drm/i915: Calculate visibility in check_plane correctly regardless of dpms.

When the crtc is configured but not active we currently clip to (0,0)x(0,0).
This results in differences in calculations depending on dpms setting.
When the crtc is enabled but not active run check_plane as if it were on,
but afterwards set plane_state->visible = false for the checks.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447945645-32005-13-git-send-email-maarten.lankhorst@linux.intel.com