drm/amdgpu: fix dma mask check in gmc_v6_0.c

This got messed up by "drm: change func to better detect wether swiotlb
is needed".

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michael D Labriola <michael.d.labriola@gmail.com>
Link: https://patchwork.freedesktop.org/patch/287070/