msm: mdss: hold mdp clock ref count when restoring ctl path

MDP control path needs to be restored whenever MDSS GDSC is turned
back on while exiting the utlra-low power state (ULPS). Hold a
reference count for the MDP clocks to ensure that there is no
unclocked register access.

Change-Id: Ia2d299d9db9c23e4c9171cbec8e376908046494b
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>