drm/msm/sde: drop crtc mutex before calling power enable

Drop the sde crtc mutex before calling to toggle the power
enable state while enabling or disabling the vblank. This is due
to the fact that if the power handle needs to call back into the
crtc, it will attempt to grab the crtc lock again causing
deadlock.

Change-Id: Ia599349dc06f3b0f8cca6a4f9d3fccf1fd7632cf
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>