drm/xe/uc: Split xe_uc_fw_init

The function does a driver specific "request firmware" step that
includes validating the input, followed by wrapping the firmware binary
into a buffer object. Split it into smaller parts.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>