drm/amd/display: Add a missing register field for HPO DP stream encoder

[Why&How]
Add the missing definition to set the register field
HBLANK_MINIMUM_SYMBOL_WIDTH

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