drm/amdgpu: Allocate coredump memory in a nonblocking way

During a GPU reset, a normal memory reclaim could block to reclaim
memory. Giving that coredump is a best effort mechanism, it shouldn't
disturb the reset path. Change its memory allocation flag to a
nonblocking one.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>