drm/amdgpu: Increase direct IB pool size

Direct IB pool is used for vce/vcn IB extra msg too. Increase its size
to AMDGPU_IB_POOL_SIZE.

v2: Squash in unused variable removal

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