drm/amd/display: fix ptr_ret.cocci warnings

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_debugfs.c:771:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Fixes: e498eb71

 ("drm/amd/display: Add support for hw_state logging via debugfs")
CC: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>