drm/i915/dpcd_bl: Cleanup intel_dp_aux_vesa_enable_backlight() a bit

Get rid of the extraneous switch case in here, and just open code
edp_backlight_mode as we only ever use it once.

v4:
* Check that backlight mode is DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD, not
  DP_EDP_BACKLIGHT_CONTROL_MODE_MASK - imirkin

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514181504.565252-4-lyude@redhat.com