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

BugLink: https://bugs.launchpad.net/bugs/1945470



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: Nirmal Abraham <nabrah@codeaurora.org>
Signed-off-by: srikanthreddy ponogoti <sponogot@codeaurora.org>
(cherry picked from commit c235a922c4a7e18ff013cd22efeb9c325e4b2f40)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>