drm/i915: Disable sprite gamma on ivb-bdw

We don't currently have any use for the sprite gamma on ivb-bdw.
Let's disable it. We already do that on skl+.

On pre-ivb there is no way to disable the sprite gamma, and it
only affects YCbCr pixel formats, whereas on ivb+ it also
affects RGB formats.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703200824.5971-3-ville.syrjala@linux.intel.com