drm/amdkfd: gfx12 context save/restore trap handler fixes

Fix LDS size interpretation: 512 bytes (>= gfx12) vs 256 (< gfx12).

Ensure STATE_PRIV.BARRIER_COMPLETE cannot change after reading or
before writing. Other waves in the threadgroup may cause this field
to assert if they complete the barrier.

Do not overwrite EXCP_FLAG_PRIV.{SAVE_CONTEXT,HOST_TRAP} when
restoring this register. Both of these fields can assert while the
wavefront is running the trap handler.

Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Lancelot Six <lancelot.six@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>