disp: msm: sde: add null check for drm file in msm_release

Drm file is not set to NULL after freeing it from drm
release. This can result in use-after-free issues in
some scenarios. Add a mutex lock and other proper null
checks to prevent such issues.

Change-Id: Ic35b0a76166b0f47a354b1737e6f4c3ac1437ed4
Signed-off-by: Althaf Neelanchirayil <aneelanc@codeaurora.org>