drm/komeda: Add debugfs node to control error verbosity

Named 'err_verbosity', currently with only 1 active bit in that
replicates the existing level - print error events once per flip.

Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107114155.54307-2-mihail.atanassov@arm.com