disp: msm: dp: add support for lane count reduction

Add support for lane count reduction as per the new requirements
for DP 1.4a during link training 2.

CRs-Fixed: 2458753
Change-Id: I58c9b6101338e8a1d1b4e3dec80f8fdf2a25ae5b
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>