drm/amdgpu: Move the mutex_lock to protect the return status of securedisplay command buffer

[Why]
Before we call psp_securedisplay_invoke(), we call
psp_prep_securedisplay_cmd_buf() to prepare and initialize the command
buffer.

However, we didn't use the mutex_lock to protect the status of command
buffer. So when multiple threads are using the command buffer, after
thread A return from psp_securedisplay_invoke() and the command buffer
status is set to SUCCESS, another thread B may call
psp_prep_securedisplay_cmd_buf() and initialize the status to FAILURE
again, and cause Thread A to get a failure return status.

[How]
Move the mutex_lock out of psp_securedisplay_invoke() to its caller to
cover psp_prep_securedisplay_cmd_buf() and the code checking the return
status of command buffer.

Signed-off-by: Alan Liu <HaoPing.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>