drm/i915: Clean up the mess around hdmi_12bpc_possible()

drm/i915: Clean up the mess around hdmi_12bpc_possible()

Move the crtc state related 12bpc checks into hdmi_12bpc_possible()
since that one already examines other parts of the crtc state.

Note that we can drop the !force_dvi check since
crtc_state->has_hdmi_sink already accounts for that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171026151405.30710-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>