drm/dp_mst: Combine redundant cases in drm_dp_encode_sideband_req()

drm/dp_mst: Combine redundant cases in drm_dp_encode_sideband_req()

Noticed this while working on adding a drm_dp_decode_sideband_req().
DP_POWER_DOWN_PHY/DP_POWER_UP_PHY both use the same struct as
DP_ENUM_PATH_RESOURCES, so we can just combine their cases.

Changes since v2:
* Fix commit message

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Juston Li <juston.li@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <hwentlan@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903215702.16984-1-lyude@redhat.com