drm/gem-shmem: Do not map s/g table by default

drm/gem-shmem: Do not map s/g table by default

The vast majority of drivers that use GEM-SHMEM helpers do not use
an s/g table for imported buffers; specifically all drivers that use
DRM_GEM_SHMEM_DRIVER_OPS. Therefore convert the initializer macro
to DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT and remove the latter. This
helps to avoid swiotbl errors, such as seen with some Aspeed systems

  ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots)

The error is caused by the system's limited DMA capabilities and can
happen with any GEM-SHMEM-based driver. It results in a performance
penalty.

In the case of vgem and vkms, the devices do not support DMA at all,
which can result in failure to map the buffer object into the kernel's
address space. [1][2] Avoiding the s/g table fixes this problem.

The other drivers based on GEM-SHMEM, imagination, lima, panfrost,
panthor, v3d and virtio, use the s/g table of imported buffers. Neither
driver uses the default initializer, so they won't be affected by
this change.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Zenghui Yu <zenghui.yu@linux.dev>
Closes: https://lore.kernel.org/dri-devel/6d22bce3-4533-4cfa-96ba-64352b715741@linux.dev/ # [1]
Reported-by: José Expósito <jose.exposito89@gmail.com>
Closes: https://lore.kernel.org/dri-devel/20250311172054.2903-1-jose.exposito89@gmail.com/ # [2]
Tested-by: Zenghui Yu <zenghui.yu@linux.dev>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://lore.kernel.org/r/20250630143537.309052-1-tzimmermann@suse.de