drm/msm/sde: moves encoder iteration complexity to DRM API

Previously CRTC commit was iterating over all encoders on DRM device
and selecting correct encoder by checking the attached CRTC on the
encoder. drm_for_each_encoder_mask API usage moves this check to DRM
API instead of managing in sde driver.

Change-Id: I33b049c22ae054fc16f9b88c61e13eb032cf9aec
Signed-off-by: Samantha Tran <samtran@codeaurora.org>