drm/msm/dp: program mainlink levels before link training

Program the mainlink levels before link training to ensure that
both the safe to exit levels and idle repetition parameters
are configured together with the link. This update is in line
with the current hardware programming guide and will ensure
better interoperability.

CRs-Fixed: 2351204
Change-Id: I9c1c4a3cf70888dbf48044201748736ab97733b7
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>