kgsl/drm: Support clock control the register control.

This patch supports clock handling api from mdss to avoid unnormal
state. When we control the register without clock, we got the problem.

Change-Id: Ie91057cf28c6cdce559c40abcbf641177213027d
Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Mathew Karimpanal <kjmathew@codeaurora.org>
(cherry picked from commit f72efb31b1b1a897dc42974c29847aef55c316aa)