drm/amd/display: Apply 60us prefetch for DCFCLK <= 300Mhz

drm/amd/display: Apply 60us prefetch for DCFCLK <= 300Mhz

[Description]
- Previously we wanted to apply extra 60us of prefetch for min DCFCLK
  (200Mhz), but DCFCLK can be calculated to be 201Mhz which underflows
  also without the extra prefetch
- Instead, apply the the extra 60us prefetch for any DCFCLK freq <=
  300Mhz

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>