drm/i915: Add support for half float framebuffers for skl+

skl+ supports fp16 pixel formats on all universal planes. Add the
necessary bits to expose that capability. The main different to
icl is that we can't scale fp16, so need to add the relevant
checks.

v2: Rebase on top of icl fp16
    Split skl+ 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/20191015193035.25982-8-ville.syrjala@linux.intel.com