drm/amdgpu: drop the non-sense firmware version check on arcturus

As the firmware versions of arcturus are different from other gfx9
ASICs. And the warning("CP firmware version too old, please update!")
caused by this check can be eliminated.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>