drm/amd/display: Add support for 6.75 GBps link rate

[Why]
The latest eDP spec version 1.5 defines a new generic link
rate of 6.75 Gbps/Lane, which needs to be supported in the driver.

[How]
Added new element to the dc_link_rate enum

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Artem Grishin <Artem.Grishin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>