drm/i915: Use the default 600ns LDO programming sequence delay

Not sure which LDO programming sequence delay should be used for the CHV
PHY, but the spec says that 600ns is "Used by default for initial
bringup", and the BIOS seems to use that, so let's do the same.

Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>