drm/msm/sde: add api to set clk_rate for a particular clock

This change adds support to set_rate on a single clock instead
of all the clocks in the module. This is required when CRTC
votes for the core clock, other clock rate change should not be
triggered. This change is required since rot_clk rate should
be set only once during bootup from the drm driver.
Rotator driver calculates the rate for rot_clk based on the session
parameters and drm driver should not touch it for any frame update.

Change-Id: I482f58708408bf05351d9b33fcdb25a3831b632b
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>
Signed-off-by: Abhijith Desai <desaia@codeaurora.org>