drm/amd/display: Modify resource allocation logic

To reduce the complexity of pipe resource allocation for different
use-cases, now we search for any free pipe sequentially rather than from
bottom up.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>