drm: fallback to dma_alloc_coherent when memory encryption is active

We can't just map any randome page we get when memory encryption is
active.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.kernel.org/patch/10850833/