drm/i915/intel_fb: Factor out convert_plane_offset_to_xy()

Factor out to a new function the logic to convert the FB plane offset to
x/y and check the validity of x/y, with the goal to make
intel_fill_fb_info() more readable.

v2: Use &fb->base instead of a drm_fb alias. (Ville)

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/20210325214808.2071517-14-imre.deak@intel.com