drm/amdgpu/gfx11: need acquire mutex before access CP_VMID_RESET v2

It's required to take the gfx mutex before access to CP_VMID_RESET,
for there is a race condition with CP firmware to write the register.

v2: add extra code to ensure the mutex releasing is successful.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>