drm: Check dp extended receiver capabilities

If present, the dp extended receiver capabilities
contain the DPRX's true capabilities. Make sure
to check and use these extended receiver
capabilities.

CRs-Fixed: 2488456
Change-Id: I32ae85a00eae95ad005ce1c00abc1c43fda1cba3
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>