drm/i915: Handle fb offset and src coordinates for cursors

drm/i915: Handle fb offset and src coordinates for cursors

The cursor plane doesn't have any kind of source offset register, so
the only form of panning possible is via a the base address register.
The alignment required by CURBASE ranges from 32B to 16KiB depending
on the platform. Let's make sure the user didn't ask for something
we can't do.

Obviously this is impossible to hit via the legacy cursor ioctl since
the src offsets are always 0, but via the plane/atomic ioctls the user
can ask for pretty much anything so we have to deal with this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-14-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>