drm/i915/display/mst: Enable virtual channel payload allocation earlier

This register was being enabled after enable TRANS_DDI_FUNC_CTL and
PIPECONF/TRANS_CONF while BSpec states that it should be set when
enabling TRANS_DDI_FUNC_CTL.

BSpec: 49190
BSpec: 22243
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107214559.77087-3-jose.souza@intel.com