drm/amd/display: Fix pixel rate divider policy for 1 pixel per cycle config

[Why]
Pixel rate dividor was not programmed correctly for 1 pixel per cycle
configuration for empty tu case.

[How]
Included check for empty tu when pixel rate dividor values were selected.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>