drm/amdgpu/gfx: Replace ARRAY_SIZE with size variable

Replace ARRAY_SIZE with size variables to support
different ASICs.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>