disp: msm: dp: reset panel object for connector when uninstalled

When panel object for a dp connector is destroyed, the instance
of dp_panel in the connector object is made NULL to prevent
further panel object dereferences.

Change-Id: I63409ab8c470f14cf9c032e373cb0c0872577504
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>