drm/i915: Fix rawclk readout for g4x

drm/i915: Fix rawclk readout for g4x

Turns out our skills in decoding the CLKCFG register weren't good
enough. On this particular elk the answer we got was 400 MHz when
in reality the clock was running at 266 MHz, which then caused us
to program a bogus AUX clock divider that caused all AUX communication
to fail.

Sadly the docs are now in bit heaven, so the fix will have to be based
on empirical evidence. Using another elk machine I was able to frob
the FSB frequency from the BIOS and see how it affects the CLKCFG
register. The machine seesm to use a frequency of 266 MHz by default,
and fortunately it still boot even with the 50% CPU overclock that
we get when we bump the FSB up to 400 MHz.

It turns out the actual FSB frequency and the register have no real
link whatsoever. The register value is based on some straps or something,
but fortunately those too can be configured from the BIOS on this board,
although it doesn't seem to respect the settings 100%. In the end I was
able to derive the following relationship:

BIOS FSB / strap | CLKCFG
-------------------------
200              | 0x2
266              | 0x0
333              | 0x4
400              | 0x4

So only the 200 and 400 MHz cases actually match how we're currently
decoding that register. But as the comment next to some of the defines
says, we have been just guessing anyway.

So let's fix things up so that at least the 266 MHz case will work
correctly as that is actually the setting used by both the buggy
machine and my test machine.

The fact that 333 and 400 MHz BIOS settings result in the same register
value is a little disappointing, as that means we can't tell them apart.
However, according to the gmch datasheet for both elk and ctg 400 Mhz is
not even a supported FSB frequency, so I'm going to make the assumption
that we should decode it as 333 MHz instead.

Cc: stable@vger.kernel.org
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Reported-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100926
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170504181530.6908-1-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
(cherry picked from commit 6f38123ecaac446312a63523b68df84ceb5a06ed)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>