drm/gma500/gma_device: Include our own header containing prototypes

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

 drivers/gpu/drm/gma500/gma_device.c:10:6: warning: no previous prototype for ‘gma_get_core_freq’ [-Wmissing-prototypes]

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/20210115181313.3431493-23-lee.jones@linaro.org