drm/msm: Deletion of unnecessary checks before two function calls

The functions framebuffer_release() and vunmap() perform also input
parameter validation. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>