drm/i915: Ignore bogus plane coordinates on SKL when the plane is not visible

When the plane is invisible we may have all sorts of bogus stuff
in the coordinates, which we must ignore or else we might fail the
plane update. This started to happen on SKL when I moved the plane
offset computation to happen in the check phase. Previously we
happily ignored it all since we never called the update_plane hook
with an invisible plane.

Cc: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Fixes: b63a16f6

 ("drm/i915: Compute display surface offset in the plane check hook for SKL+")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98258


Testcase: igt/pm_rpm/legacy-planes
Testcase: igt/pm_rpm/universal-planes
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478550057-24864-3-git-send-email-ville.syrjala@linux.intel.com
(cherry picked from commit a5e4c7d0

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>