drm/i915: refactor cursor code out of i915_display.c

This file is a monster, let's start simple, the cursor plane code
seems pretty standalone, and splits out easily enough.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: cleaned up intel_cursor.h a bit.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201221090449.8288-4-jani.nikula@intel.com