drm/i915: HSW FBC WaFbcDisableDpfcClockGating

Display register 46500h bit 23 must be set to 1b for the entire time that
Frame Buffer Compression is enabled.

v2: Ville suggested to enable it back when disabling fbc to avoid wasting
    power.

v3: RMW to preserve other bits (by Ville)
v4: Fix from Ville: sed &/| at RMW
v5: Too far on sed.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Insert missing space that checkpatch spotted.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>