drm/amdgpu: remove ctx->lock

KMD reports a warning on holding a lock from drm_syncobj_find_fence,
when running amdgpu_test case “syncobj timeline test”.

ctx->lock was designed to prevent concurrent "amdgpu_ctx_wait_prev_fence"
calls and avoid dead reservation lock from GPU reset. since no reservation
lock is held in latest GPU reset any more, ctx->lock can be simply removed
and concurrent "amdgpu_ctx_wait_prev_fence" call also can be prevented by
PD root bo reservation lock.

call stacks:
=================
//hold lock
amdgpu_cs_ioctl->amdgpu_cs_parser_init->mutex_lock(&parser->ctx->lock);
…
//report warning
amdgpu_cs_dependencies->amdgpu_cs_process_syncobj_timeline_in_dep \
->amdgpu_syncobj_lookup_and_add_to_sync -> drm_syncobj_find_fence \
-> lockdep_assert_none_held_once
…
amdgpu_cs_ioctl->amdgpu_cs_parser_fini->mutex_unlock(&parser->ctx->lock);

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