drm: Extract drm_encoder.[hc]

Same treatment as before. Only hiccup is drm_crtc_mask, which
unfortunately can't be resolved until drm_crtc.h is less of a monster.
Untangle the header loop with a forward declaration for that static
inline.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-1-daniel.vetter@ffwll.ch