drm/dp/mst: Use memchr_inv() instead of memcmp() against a zeroed array

We have memch_inv(), so no need to memcmp() against a zeroed temp array.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170712155254.26455-1-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>