drm/arm/malidp: Set the output_depth register in modeset

One needs to store the value of the OUTPUT_DEPTH that one has parsed from
device tree, so that it can be restored on system resume. This value is
set in the modeset function as this gets reset when the system suspends.

Signed-off-by: Ayan Kumar Halder <ayan.halder@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>