drm/i915: Move all FBC w/as to .init_clock_gating()

drm/i915: Move all FBC w/as to .init_clock_gating()

Some platforms apply the FBC w/as in .init_clock_gating(), some
in fbc_activate(). Move them all to .init_clock_gating() for
consistentce. Also safer since we don't have to worry about the
RMWs clashing with any other runtime use of the same registers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200708131223.9519-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>