disp: msm: vote for reg bus during rsc pm_runtime events

Add reg bus vote separate for RSC on top of the SDE
driver vote during the pm_runtime suspend/resume
events to avoid register access issues.

Change-Id: Ifbaead747b7b10db107696c8cce8e1ecfae5e0f8
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>