msm_fb: Remove the extra MDP clock enable in writeback_commit

There is an extra MDP clock enable call in writeback commit
which causes the MDP clock to be enabled always once a WFD
session is setup. Remove this additional clock vote to make
sure the clock enable/disable calls are balanced.

Change-Id: I8b101f3eb76fe00ecf6d3967d0bf113146447db3
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>