disp: msm: sde: cache encoder_mask for vblank_work

There is a race between disable commit swap-state on display
thread and vblank work on event thread which may skip applying
the vblank vote if the encoder_mask has already been cleared.
To avoid the race, use a cached encoder_mask that is gauranteed
to be valid between the vblank_on and vblank_off calls.

Also, vblank queue work is initializing a variable that's never
used and msm_disable_outputs has 2 instances of pointers to old
CRTC state. Remove this dead code.

Change-Id: I5e4a482b8f067e272a2aef5afa08cc0e1ab89434
Signed-off-by: Steve Cohen <cohens@codeaurora.org>