drm/i915/intel_fb: Factor out calc_plane_aligned_offset()

Factor out to a new function the logic to convert the FB plane x/y
values to a tile size based offset and new x/y relative to this offset.
This makes intel_fill_fb_info() and intel_plane_remap_gtt() somewhat
more readable.

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/20210325214808.2071517-15-imre.deak@intel.com