msm: mdss: dp: add support for link re-training

Add support for link re-training after the main link is already
trained and active. Parse the requested lane count and link
bandwidth from the DPCD when hpd_irq is high, and re-train
the main link once the display, and therefore timing generator,
has been turned off.

CRs-Fixed: 1076516
Change-Id: Ifa1b609c532aa601f30e334e87a768bdda78958d
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>