drm/amd/display: Fix unused-but-set-variable warning

Fix the kernel test robot warning below:

drivers/gpu/drm/amd/amdgpu/../display/dmub/inc/dmub_cmd.h:2893:12:
warning: variable 'temp' set but not used [-Wunused-but-set-variable]

Replaced the assignment to the unused temp variable with READ_ONCE()
macro to flush the writes. READ_ONCE() helps avoid the use of
volatile and makes it obvious from the code that the read here is
intentional. Also verified on x86 that the generated code is exactly the
same as before.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Aashish Sharma <shraash@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>