drm/i915: Add support for half float framebuffers on snb sprites

snb supports fp16 pixel formats on the sprite planes. Expose that
capability. Nothing special needs to be done, it just works.

v2: Rebase on top of icl fp16
    Split snb+ sprite bits into a separate patch

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015193035.25982-11-ville.syrjala@linux.intel.com