drm/msm/sde: enable sde clocks during debugfs register read

SDE clocks need to be enabled when attempting to read h/w
registers through the debugfs interface. The clocks themselves
are reference counted, so it's safe to enable/disable them
around the debugfs access.

Change-Id: I937b29e327799f78b821d4beb4f93504bea28fdc
Signed-off-by: Clarence Ip <cip@codeaurora.org>