drm/arc: Stop consulting plane->fb

We want to stop using plane->fb with atomic driver, so stop looking at
it.

I have no idea what this code is trying to achieve. There is no
corresponding check in the enable path. Also since
arc_pgu_set_pxl_fmt() will anyway oops if there is no fb I'm going
to assuming that I can just remove the check entirely. There seems
to be a general shortage of .atomic_check() in this driver...

Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405195035.24722-1-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alexey Brodkin <abrodkin@synopys.com>