drm/i915/dpcd_bl: Move VESA backlight enabling code closer together

No functional changes, just move set_vesa_backlight_enable() closer to it's
only caller: intel_dp_aux_vesa_enable_backlight().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514181504.565252-6-lyude@redhat.com