drm/gma500/gem: Do not rely on consumers to include drm/drm_device.h before gem.h

drm/gma500/gem: Do not rely on consumers to include drm/drm_device.h before gem.h

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/gma500/gem.h:13:57: warning: ‘struct drm_device’ declared inside parameter list will not be visible outside of this definition or declaration

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-32-lee.jones@linaro.org