drm/i915: Fix 400 MHz FSB readout on elk

drm/i915: Fix 400 MHz FSB readout on elk

Looks like elk redefines some of the CLKCFG FSB values to
make room for 400 MHz FSB. The setting overlaps with one of
the 266MHz settings (which is even documented in the ctg docs,
and cofirmed to be correct on my ctg). So we limit the special
case to elk only.

Though it might also be that we have some kind of desktop vs.
mobile difference going on here as eg. both g35 and elk
use 0x0 for the 266 MHz setting, vs. 0x6 used by ctg). The
g35 doesn't let me select 400MHz for the FSB strap so can't
confirm which way it would go here. But anyways as it seems
only elk has the 400MHz option we shouldn't lose anything
by limiting the special case to it alone.

My earlier experiments on this appear to have been nonsense as
the comment I added claims that FSB strap of 400MHz results in
a value of 0x4, but I've now retested it and I definitely get a
value of 0x6 instead. So let's remove that bogus comment.

v2: s/_ELK/_ALT/ in the define in anticipation of a full
    mobile vs. desktop CLKCFG split

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514123838.3017-1-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>