drm/i915: Move the SKL+ zero constant alpha handling

drm/i915: Move the SKL+ zero constant alpha handling

Let's run through the entire plane check even when the plane
is invisible due to zero constant alpha. This makes for more
consistent behaviour since we check the src/dst coordinates,
stride etc. against the hardware limits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018195921.9898-2-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>