drm/gpusvm: fix hmm_pfn_to_map_order() usage

drm/gpusvm: fix hmm_pfn_to_map_order() usage

Handle the case where the hmm range partially covers a huge page (like
2M), otherwise we can potentially end up doing something nasty like
mapping memory which is outside the range, and maybe not even mapped by
the mm. Fix is based on the xe userptr code, which in a future patch
will directly use gpusvm, so needs alignment here.

v2:
  - Add kernel-doc (Matt B)
  - s/fls/ilog2/ (Thomas)

Reported-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://lore.kernel.org/r/20250828142430.615826-11-matthew.auld@intel.com