drm/vc4: crtc: Fix redundant variable assignment

The variable is assigned twice to the same value. Let's drop one.

Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220203151151.1270461-1-maxime@cerno.tech