drm/gma500: make local header files more self-contained

Add proper forward declarations to minimize dependencies on
other header files.

Just add enough that we can safely include all header files in
alphabetically order in relevant files.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190519195526.3422-4-sam@ravnborg.org

drivers/gpu/drm/gma500/cdv_device.h

@@ -15,6 +15,10 @@
  * 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+struct drm_crtc;
+struct drm_device;
+struct psb_intel_mode_device;
+
 extern const struct drm_crtc_helper_funcs cdv_intel_helper_funcs;
 extern const struct drm_crtc_funcs cdv_intel_crtc_funcs;
 extern const struct gma_clock_funcs cdv_clock_funcs;