drm/dp_mst: Refactor drm_dp_update_payload_part1()

This:

- Adds local variables in the first loop, instead of using array indices
  everywhere
- Adds an early continue to reduce the indent level in the second loop

There should be no functional changes here

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Juston Li <juston.li@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181214012604.13746-3-lyude@redhat.com