drm/i915: Add intel_crtc_cursor_set_obj() to set cursor buffer (v2)

Refactor cursor buffer setting such that the code to actually update the
cursor lives in a new function, intel_crtc_cursor_set_obj(), and takes
a GEM object as a parameter.  The existing legacy cursor ioctl handler,
intel_crtc_cursor_set() will now perform the userspace handle lookup and
then call this new function.

This refactoring is in preparation for the universal plane cursor
support where we'll want to update the cursor with an actual GEM buffer
object (obtained via drm_framebuffer) rather than a userspace handle.

v2:  Drop obvious kerneldoc and replace with note about function's
     reference consumption

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Pallavi <G&lt;pallavi.g@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>