drm/msm/sde: cache crtc in sde encoder

DRM framework clears the crtc and connector links during normal display
commit work in drm_atomic_helper_update_legacy_modeset_state. This leads
to race condition on accessing the crtc and connector pointer.
This change caches the crtc pointer on encoder enable and clears it on
disable.

Change-Id: I0327033a9a7b9070e8acd437e804a8fe0fd1bb97
Signed-off-by: Harsh Sahu <hsahu@codeaurora.org>