drm/i915/display: Convert intel_fb_modifier_to_tiling as non-static

We are about to split i915 specific code from intel_fb.c. Convert
intel_fb_modifier_to_tiling as non-static to allow calling it from split
code.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231203114840.841311-3-jouni.hogander@intel.com