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: b2081525 ("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.comReviewed-by: Matt Roper <matthew.d.roper@intel.com>
(cherry picked from commit 968bf969)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_sprite.c

@@ -494,7 +494,7 @@ skl_program_plane(struct intel_plane *plane,
 
 	keymax = (key->max_value & 0xffffff) | PLANE_KEYMAX_ALPHA(alpha);
 
-	keymsk = key->channel_mask & 0x3ffffff;
+	keymsk = key->channel_mask & 0x7ffffff;
 	if (alpha < 0xff)
 		keymsk |= PLANE_KEYMSK_ALPHA_ENABLE;