drm/amdgpu: retire AMDGPU_REGS_KIQ flag

all the register access through kiq is redirected
to amdgpu_kiq_rreg/amdgpu_kiq_wreg

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>