drm/i915: refactor some crtc code out of intel display. (v2)

There may be more crtc code that can be pulled out, but this
is a good start.

v2: move plane before this.

Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: cleaned up intel_crtc.h a bit.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/eacbe964f90d189c5940c12af5e09091b37a19c3.1610622609.git.jani.nikula@intel.com