drm/amd/display: always use mpc factor of 2 for stereo timings

[why]
In the new pipe resource management logic, the special handling for
stereo timings is missing.
This commit implements the same stereo timings handling as old
pipe resource management code.

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>