drm/arc: Drop crtc check in arc_pgu_update

drm/arc: Drop crtc check in arc_pgu_update

It's redundant, drm core guarantees that state->fb is set iff
state->crtc is set.

v2: I had a misconception about simple helpers here and thought they
filter this out. They don't. Issue reported by Eugeniy.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-10-daniel.vetter@ffwll.ch