drm/amd/display: Consolidate two-pixels-per-container check

[why]
The condition to check for two pixels per containter has become rather
long and is used in number of places.

[how]
Move the check to a helper function.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>