drm/zte: Stop consulting plane->crtc

We want to get rid of plane->crtc on atomic drivers. Stop looking at it.

v2: Use old_state->crtc (Maarten)
v3: s/fb/crtc/ in commit message to actually match the patch (Shawn)

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326121442.32009-1-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>