drm/gpusvm: Allow device pages to be mapped in mixed mappings after system pages

drm/gpusvm: Allow device pages to be mapped in mixed mappings after system pages

The current code rejects device mappings whenever system pages have
already been encountered. This is not the intended behavior when
allow_mixed is set.

Relax the restriction by permitting a single pagemap to be selected when
allow_mixed is enabled, even if system pages were found earlier.

Fixes: bce13d6ecd6c ("drm/gpusvm, drm/xe: Allow mixed mappings for userptr")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patch.msgid.link/20260130194928.3255613-3-matthew.brost@intel.com