drm/i915/uc: Separate firmware selection and preparation

intel_{h,g}uc_init_fw selects correct firmware and then triggers it's
preparation (fetch + initial parsing).

This change separates out select steps, so those can be called by
the sanitize_options().

Then, during the init_fw(), we prepare the firmware if the firmware was
selected.

Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>