drm/i915: Read out hrawclk on all gen3+ platforms

I've checked a bunch of gen3/4 machines and all seem to have
consistent FSB frequency information in the CLKCFG register.
So let's read out hrawclk on all gen3+ machines. Although
apart from g4x/pnv aux/pps dividers we only really need this
for for i965g/gm cs timestamp increment.

The CLKCFG memory clock values seem less consistent but we
don't care about those here.

For posterity here's a list of CLKCFG vs. FSB dumps from
a bunch of machines (only missing lpt for a full set):
machine CLKCFG     FSB
alv1    0x00001411 533
alv2    0x00000420 400 (Chris)
gdg1    0x20000022 800
gdg2    0x20000022 800
cst     0x00010043 666
blb     0x00002034 1333
pnv1    0x00000423 666
pnv2    0x00000433 666
965gm   0x00004342 800
946gz   0x00000022 800
965g    0x00000422 800
g35     0x00000430 1066
        0x00000434 1333
ctg1    0x00644056 1066
ctg2    0x00644066 1066
elk1    0x00012420 1066
        0x00012424 1333
        0x00012436 1600
        0x00012422 800
elk2    0x00012040 1066

For the mobile parts the chipset docs generally have these
documented to some degree (alv being the exception).

The two settings w/o any evidence are 0x5=400MHz on desktop
and 0x7=1333MHz on mobile. Though the mobile 1333MHz case
probably doesn't even exist since ctg is only documented
to go up to 1066MHz.

v2: Fix 400mhz readout for Chris's alv/celeron machine
    Do a clean mobile vs. dekstop split since that's really
    what seems to be going on

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514123838.3017-3-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>

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