drm/msm: fix memleak on release

If a process is interrupted while accessing the "gpu" debugfs file and
the drm device struct_mutex is contended, release() could return early
and fail to free related resources.

Note that the return value from release() is ignored.

Fixes: 4f776f45

 ("drm/msm/gpu: Convert the GPU show function to use the GPU state")
Cc: stable <stable@vger.kernel.org>     # 4.18
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010131333.23635-2-johan@kernel.org