drm/amd/display: Remove unnecessary divider update flag

drm/amd/display: Remove unnecessary divider update flag

[Why]
When transitioning from 640x480 at RBRx1 to HBR3x1,
both output pixel mode and pixel rate divider should update.
The needs_divider_update flag was only for 8b10b and 128b132b transition.

[How]
Remove needs_divider_update flag.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Cruise Hung <Cruise.Hung@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>