drm/amd/display: Conditionally enable 6.75 GBps link rate

[Why]
The 6.75 GBps link rate is part of the new eDP specification
version 1.5 is going to be supported in the future.

Since this standard is very new and there are no existing 6.75 GBps
panels on the market yet, we should put a condition in the driver
on enabling this feature until we can validate it with real hardware.

[How]
- Add boolean flag support_eDP1_5 in struct dc_debug_options.
- Enable the 6.75 link rate in reduce_link_rate(...) only when
  the flag is true.

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>