drm/msm: clean up display thread

Since there are no clients using these threads,
cleaning it up.

changes in v2:
	- switch all the dependent clients to use system wq
	  before removing the disp_threads (Sean Paul)
changes in v3:
	- none
changes in v4:
	- none
changes in v5:
	- Rebase on latest tip with [1] (Sean Paul)

[1] https://patchwork.freedesktop.org/patch/255105/



Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>