drm/i915: Fix skl srckey mask bits

drm/i915: Fix skl srckey mask bits

We're incorrectly masking off the R/V channel enable bit from
KEYMSK. Fix it up.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Fixes: 28846717a3c5 ("drm/i915: Add plane alpha blending support, v2.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125183846.28755-1-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
(cherry picked from commit 968bf969b47df2481022b9a05eaab02948eec088)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>