drm/i915/dsi: fuse dsi_send_pkt_payld() and add_payld_to_queue()

Having two functions for this seems like excess duplication and
parameter juggling. Merge them together.

While at it, drop the extra error message, as wait_for_payload_credits()
already prints an error, and switch from incidental -EPERM (i.e. -1) to
actual error codes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f74f7462a36e76070db6b4c01616d0eb663b9938.1633000838.git.jani.nikula@intel.com