drm/amdgpu: adjust default dispclk (v2)

Set the default to 600Mhz if it's not set in the bios,
and bump the default to 600Mhz if it's lower than that.

Port of radeon commit:
9368931d

v2: clean up the code a bit

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=91896



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>