msm: mdss: issue panic if pipe halt is requested while in use

Pipe halt is requested when pipe is no longer in use or when pipe
is initialized to be used. Now if there is a mismatch between SW state
of pipe being use and HW state then it can lead undesirable issues.
Issue panic if halt is requested while pipe is still in use by HW.

Change-Id: I11d20611eeb3cb991a4dc3e580fb83d86289b68d
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>