drm/amd/display: Source minimum HBlank support

[Why]
Some sink devices wish to have access to the minimum HBlank supported by
the ASIC.

[How]
Make the ASIC minimum HBlank available in Source Device information
address 0x340.

Signed-off-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>