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>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>