drm/i915/fb: Factor out functions to remap contiguous FB obj pages

Factor out functions needed to map contiguous FB obj pages to a GTT/DPT
VMA view in the next patch.

While at it s/4096/I915_GTT_PAGE_SIZE/ in add_padding_pages().

No functional changes.

v2: s/4096/I915_GTT_PAGE_SIZE/ (Matthew)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211026225105.2783797-4-imre.deak@intel.com