drm/i915/display: Remove unused declarations of intel_io_*

drm/i915/display: Remove unused declarations of intel_io_*

Declarations for both intel_io_mmio_fw_write and intel_io_reg_write
were added with commit 01389846f7d6 ("drm/i915: Plumb 'dsb' all way to
the plane hooks"), but they never got used. Let's remove them.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://lore.kernel.org/r/20250717-drop-unused-intel_io-declarations-v1-1-bdea2c749571@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>