drm/i915: Populate gamma_mode for all platforms

On pre-HSW gamma mode is configured via PIPECONF. The bits are
the same except shifted up, so we can reuse just store them in
crtc_state->gamma_mode in the HSW+ way, allowing us to share
some code later.

v2: Allow fastboot with gamma_mode changes (Maarten)
    Add space around the '<<' in the reg macro
    Deal with HAS_GMCH

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190207202146.26423-2-ville.syrjala@linux.intel.com