drm/amdgpu/dma_buf: fix page_link check

The page_link lower bits of the first sg could contain something like
SG_END, if we are mapping a single VRAM page or contiguous blob which
fits into one sg entry. Rather pull out the struct page, and use that in
our check to know if we mapped struct pages vs VRAM.

Fixes: f44ffd67 ("drm/amdgpu: add support for exporting VRAM using DMA-buf v3")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.8+
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>