msm: mdss: remove unnecessary lock in checking DSI status

Since DSI status checking is purely DSI operation, it's not
necessary to acquire ov_lock in case of video mode, otherwise
this function could block other overlay functions, therefore
introducing extra delay.

CRs-fixed: 714531
Change-Id: I5051ff3495d516b405cd3bf5dc32ea712187975e
Signed-off-by: Ray Zhang <rayz@codeaurora.org>