drm: xilinx: dp: Add delay after Monitor wake up

drm: xilinx: dp: Add delay after Monitor wake up

Some monitors require delay to fully wake up. Otherwise, it may
result in some error such as training failure.

Delay of 4 msec was not specified in the spec, but found from
experimentation (ex, no failure for 20 times or more). Thus,
this setting is exposed as module parameter so that user can
change if needed.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>