drm/radeon: fix PLLs on RS880 and older v2

This is a workaround for RS880 and older chips which seem to have
an additional limit on the minimum PLL input frequency.

v2: fix signed/unsigned warning

bugs:
https://bugzilla.kernel.org/show_bug.cgi?id=91861
https://bugzilla.kernel.org/show_bug.cgi?id=83461



Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org