drm/amd/display: Remove PSR functions in Linux

NULL check issue found by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:1976
dc_link_setup_psr() warn: variable dereferenced before check 'link' (see
line 1970)

We don't use these functions so might as well remove them.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>