drm/msm/dp: fix error checks in hotplug module

Fix the error checks and return values in the hotplug module so
that the upper layers can correctly handle failures from the
module.

Change-Id: I0646dcb55515fb163f3dba1c4de60c75818ce392
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>