drm/amd/display: Change HDR_MULT check

[Why]
Currently we require HDR_MULT >= 1.0
There are scenarios where we need < 1.0

[How]
Only guard against 0 - it will black-screen image.
It is up to higher-level logic to decide what HDR_MULT
values are allowed in each particular case.

Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>