drm: Fix compilation warning caused by static inline forward declaration

The drm_crtc_mask() function used in <drm/drm_encoder.h> is a static
inline defined in <drm/drm_crtc.h>. If the first header is included in a
compilation unit without the second one, the following compilation
warning will be issued.

In file included from <linux>/drivers/gpu/drm/drm_bridge.c:29:0:
<linux>/include/drm/drm_encoder.h:192:95: warning: ‘drm_crtc_mask’ used but never defined
 static inline uint32_t drm_crtc_mask(const struct drm_crtc *crtc);

Fix this by including the header defining the function instead of using
a forward declaration.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1481709550-29226-3-git-send-email-laurent.pinchart+renesas@ideasonboard.com