msm: sde: allow rot_clk_src to be absent for sde rotator

The rot_clk_src clock request is no longer required
for newer targets. The rotator now supports targets
that do not have rot_clk_src defined and will use
rot_clk for clock requests on those targets.

CRs-Fixed: 2040987
Change-Id: Ic64ff535c4845ab8dc39159cf0e61094837f26c5
Signed-off-by: Clarence Ip <cip@codeaurora.org>