drm/omap: Restrict compile testing to PAGE_SIZE less than 64KB

Prior to commit dc6fcaa ("drm/omap: Allow build with
COMPILE_TEST=y"), it was only possible to build the omapdrm driver with
a 4KB page size. After that change, when the PAGE_SIZE is 64KB or
larger, clang points out that the driver has some assumptions around the
page size implicitly by passing PAGE_SIZE to a parameter with a type of
u16:

  drivers/gpu/drm/omapdrm/omap_gem.c:758:7: error: implicit conversion from 'unsigned long' to 'u16' (aka 'unsigned short') changes value from 65536 to 0 [-Werror,-Wconstant-conversion]
    757 |                 block = tiler_reserve_2d(fmt, omap_obj->width, omap_obj->height,
        |                         ~~~~~~~~~~~~~~~~
    758 |                                          PAGE_SIZE);
        |                                          ^~~~~~~~~
  arch/powerpc/include/asm/page.h:25:34: note: expanded from macro 'PAGE_SIZE'
     25 | #define PAGE_SIZE               (ASM_CONST(1) << PAGE_SHIFT)
        |                                  ~~~~~~~~~~~~~^~~~~~~~~~~~~
  drivers/gpu/drm/omapdrm/omap_gem.c:1504:44: error: implicit conversion from 'unsigned long' to 'u16' (aka 'unsigned short') changes value from 65536 to 0 [-Werror,-Wconstant-conversion]
   1504 |                         block = tiler_reserve_2d(fmts[i], w, h, PAGE_SIZE);
        |                                 ~~~~~~~~~~~~~~~~                ^~~~~~~~~
  arch/powerpc/include/asm/page.h:25:34: note: expanded from macro 'PAGE_SIZE'
     25 | #define PAGE_SIZE               (ASM_CONST(1) << PAGE_SHIFT)
        |                                  ~~~~~~~~~~~~~^~~~~~~~~~~~~
  2 errors generated.

As there is a lot of use of a u16 type throughout this driver and it
will only ever be run on hardware that has a 4KB page size, just
restrict compile testing to when the page size is less than 64KB (as no
other issues have been discussed and it keeps compile testing relatively
more available).

Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Dave Airlie <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/20240620-omapdrm-restrict-compile-test-to-sub-64kb-page-size-v1-1-5e56de71ffca@kernel.org

drivers/gpu/drm/omapdrm/Kconfig

@@ -2,7 +2,7 @@
 config DRM_OMAP
 	tristate "OMAP DRM"
 	depends on DRM && OF
-	depends on ARCH_OMAP2PLUS || COMPILE_TEST
+	depends on ARCH_OMAP2PLUS || (COMPILE_TEST && PAGE_SIZE_LESS_THAN_64KB)
 	select DRM_KMS_HELPER
 	select FB_DMAMEM_HELPERS_DEFERRED if DRM_FBDEV_EMULATION
 	select VIDEOMODE_HELPERS