drm/amdgpu: reject oversized IBs with per-ring packet limits

drm/amdgpu: reject oversized IBs with per-ring packet limits

On GFX rings, amdgpu_cs_p2_ib() passed user-supplied ib_bytes through
to ib->length_dw without a limit, while ring_emit_ib() encodes length
into packet fields. Oversized values can corrupt adjacent control bits
and destabilize command submission.

Add a per-ring IB packet size limit helper and reject command
submissions exceeding the corresponding dword limit before IB
allocation. Use the documented 20-bit limit for GFX/compute/SDMA/VPE,
and apply the MM fallback limit for other ring types.

Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 7f48fa2cf62e3fa6c9c3870aa74988f773247e52)
Cc: stable@vger.kernel.org