drm: Don't include <drm/drm_encoder.h> in <drm/drm_crtc.h>

<drm/drm_crtc.h> used to define most of the in-kernel KMS API. It has
now been split into separate files for each object type, but still
includes most other KMS headers to avoid breaking driver compilation.

As a step towards fixing that problem, remove the inclusion of
<drm/drm_encoder.h> from <drm/drm_crtc.h> and include it instead where
appropriate. Also remove the forward declarations of the drm_encoder and
drm_encoder_helper_funcs structures from <drm/drm_crtc.h> as they're not
needed in the header.

<drm/drm_encoder.h> now has to include <drm/drm_mode.h> and contain a
forward declaration of struct drm_encoder in order to allow including it
as the first header in a compilation unit.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sinclair Yeh <syeh@vmware.com> # For vmwgfx
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1481709550-29226-2-git-send-email-laurent.pinchart+renesas@ideasonboard.com