drm/dp: cache edid for mst physical output ports

The current implementation is caching the edid during add_port
only for logical output ports and depending on get_edid api
to read edid for physical output ports. get_edid for physical
ports is executed only when get_connector ioctl is performed
by drm client.

The get_connector ioctl is locking the operation using mode_config
lock/unlock and get_edid for mst port may take couple of seconds
due to mst sideband operations. This lock/unlock is not allowing
atomic_commit ioctl for other active displays which results in
screen freeze.

This change caches the edid for mst physical ports during the
add_port operation to avoid executing get_edid as part of get_connector
ioctl to complete the ioctl operation as quick as possible. This
unblocks atomic_commit ioctl for other active displays to minimize
the screen freeze issue.

Change-Id: I993d89b8bd84f776494d72bc8ceb48eb62d2322b
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>