drm/msm/sde : cache crtc in sde phys wb encoder

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

Change-Id: I748567102739328e612a5ae66c6c16c89c020234
Signed-off-by: Harsh Sahu <hsahu@codeaurora.org>