disp: msm: sde: limit encoders attached to crtc during trusted-ui

Add checks to block transition to trusted-vm when multiple encoders
are attached to a crtc. This ensures, concurrent writeback is
disabled before trusted-ui usecase. Additionally, avoid enabling /
disabling the IRQs associated with the cwb encoder during all
the transitions to avoid any unbalanced calls.

Change-Id: I022077018ac9b7dfb62506cfaddcb60cb8b35ed8
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>