drm/i915: Add support for half float framebuffers for ivb+ sprites

ivb+ supports fp16 pixel formats on the sprite planes planes. Expose
that capability.

On ivb/hsw fp16 scanout is slightly busted. The output from the plane
will have 1/4 the expected value. For the sprite plane we can fix that
up with the plane gamma unit. This was fixed on bdw.

v2: Rebase on top of icl fp16
    Split the ivb+ sprite birs into a separate patch
v3: Move ivb_need_sprite_gamma() check one level up so that
    we don't waste time programming garbage into he gamma registers

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/20191015193035.25982-10-ville.syrjala@linux.intel.com