drm/i915/glk: Refactor handling of PLANE_COLOR_CTL for GLK+

Since GLK, some plane configuration settings have moved to the
PLANE_COLOR_CTL register. Refactor handling of the register to work like
PLANE_CTL. This also allows us to fix the set/read of the plane Alpha
Mode for GLK+.

v2: Adjust ordering of platform checks to be newest->oldest, drop
redundant comment about alpha blending. (Ville)

v3: Move Alpha Mode bits out of skl_plane_ctl_format into
skl_plane_ctl_alpha, and drop glk_plane_ctl_format, drop initialization
of state->color_ctl on platforms that don't use it, and drop color_ctl
local var. (Ville)

v4: Consolidate skl_plane_ctl_format switch statement on formats that
return the same settings. (Ville)

Signed-off-by: James Ausmus <james.ausmus@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113181128.2926-1-james.ausmus@intel.com


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>