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 <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: e20dfd27 ("drm/mgag200: Add support for G200 desktop cards")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Egbert Eich <eich@suse.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200807180547.GA923146@ravnborg.org