drm/i915/sprite: move sprite_name() to intel_sprite.c

Move sprite_name() where its only user is, and convert it to a function,
removing the implicit dev_priv usage.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231011202259.1090131-1-jani.nikula@intel.com