drm/i915: Relocate intel_bw_crtc_update()

intel_bw_crtc_update() is only used by the readout path, so relocate
the function next its only caller. Easier to read the code when related
things are nearby.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306163420.3961-19-ville.syrjala@linux.intel.com