drm/i915: Add support for FBs requiring a POT stride alignment

An upcoming platform has a restriction that the FB stride must be
power-of-two aligned. To support framebuffer layouts that are not in
this layout add a logic that pads the tile rows to the POT aligned size.

The HW won't read the padding PTEs, so these don't have to point to an
allocated address, or even have their valid flag set. So use a NULL PTE
instead for instance the scratch page, which is simple and keeps the SG
table compact.

v2:
- Simplify plane_view_dst_stride(). (Ville)
- Pass pitch_tiles as unsigned int.
v3:
- Drop unintentional s/plane_state->rotation/plane_config->rotation/
  change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325214808.2071517-24-imre.deak@intel.com