drm/mgag200: fix build on alpha arch

When building imgag200 for the alpha architecture it fails like this:
mgag200_drv.c:233:9: error: implicit declaration of function ‘vmalloc’
  233 |  bios = vmalloc(size);
      |         ^~~~~~~
      |         kmalloc

When building for other architectures vmalloc.h is pulled in via some
other header file - for example asm-generic/io.h.
Use an explicit include of vmalloc.h to fix the build.

Signed-off-by: Sam Ravnborg <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Fixes: e20dfd2 ("drm/mgag200: Add support for G200 desktop cards")
Cc: Thomas Zimmermann <[email protected]>
Cc: Egbert Eich <[email protected]>
Cc: Takashi Iwai <[email protected]>
Cc: Lyude Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

drivers/gpu/drm/mgag200/mgag200_drv.c

@@ -9,6 +9,7 @@
 #include <linux/console.h>
 #include <linux/module.h>
 #include <linux/pci.h>
+#include <linux/vmalloc.h>
 
 #include <drm/drm_drv.h>
 #include <drm/drm_file.h>