drm/pagemap: Drop source_peer_migrates flag and assume true

drm/pagemap: Drop source_peer_migrates flag and assume true

All current users of DRM pagemap set source_peer_migrates to true during
migration, and it is unclear whether any user would ever want to disable
this for performance reasons or for features such as compression. It is
also questionable whether this flag could be made to work with
high-speed fabric mapping APIs.

Drop the flag and make DRM pagemap unconditionally assume that
source_peer_migrates is true.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patch.msgid.link/20260410205929.3914474-3-matthew.brost@intel.com