drm/panthor: Don't create a file offset for NO_MMAP BOs

drm/panthor: Don't create a file offset for NO_MMAP BOs

Right now the DRM_PANTHOR_BO_NO_MMAP flag is ignored by
panthor_ioctl_bo_mmap_offset(), meaning BOs with this flag set can
have a file offset but can't be mapped anyway, because
panthor_gem_mmap() will filter them out.

If we error out at mmap_offset creation time, we can get rid of
panthor_gem_mmap() and call drm_gem_shmem_object_mmap directly, and
we get rid of this inconsistency of having an mmap offset for a
BO that can never be mmap-ed.

Changes in v2:
- Get rid of panthor_gem_mmap()
- Get rid of the Fixes tag and adjust the commit message accordingly
- Return ENOPERM instead of EINVAL

Changes in v3:
- Don't leak the BO ref
- Add R-bs

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Link: https://lore.kernel.org/r/20250417121942.3574111-1-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>