drm/amdgpu: Make amdgpu_cs_parser_init static (v2)

The function is called only once inside the .c file.
v2: update the commit message (Michel)

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>