drm/amd/display: Add debug out when viewport too small

[why] It helps debugging display setup issues

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>