drm/msm: enable/disable sde during driver open and close

Clocks for the sde driver should only be enabled when the
device is actually in use, so change the driver to enable
and disable the clocks during driver open/close rather
than enabling them initially and never disabling them.

Change-Id: I19b98ec1ed86c08885ef6df1d05c7f692952c5b9
Signed-off-by: Clarence Ip <cip@codeaurora.org>