drm/i915/migrate: don't check the scratch page

The scratch page might not be allocated in LMEM(like on DG2), so instead
of using that as the deciding factor for where the paging structures
live, let's just query the pt before mapping it.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206112539.3149779-1-matthew.auld@intel.com