drm/msm/sde: fix for buffer corruption issues with mst connectors

Current implementation of get_output_fence is depending on
atomic_state to extract the connector details. There is a
possibility to have no connectors in atomic_state if no
connector property is processed by the time of executing
get_output_fence property and this results in wrong fence
handling.

This change depends on crtc encoder_mask field to extract
the drm_encoder tied to drm_crtc and extracts the panel
mode, video or command, information from sde_encoder object.

Change-Id: I3145b543382dcac038f38ee2769309b179395b3c
Signed-off-by: Samantha Tran <samtran@codeaurora.org>