drm/i915: Make the LPT iclkip 20MHz case more generic

drm/i915: Make the LPT iclkip 20MHz case more generic

The reason for spcial casing 20MHz in the iclkip calculations is that
it would overflow the 7 bit divisor value. Let's rewrite the special
case to check for just that, and bump up auxdiv when needed. This makes
the code work for freqeuencies close to but not exactly 20MHz. The real
lower limit for auxdiv=0 is actually:
172800000/(0x7f+2)*64)=~20930 kHz, and below that we must resort to
auxdiv=1.

Actually this is all very theoretical since we limit the dotclock to
min 25MHz with CRT on all platforms. 25Mhz is actually the documented
limit in Bspec, so it seems we ought to never need to worry about the
auxdiv=1 case. But no harm in having it.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455738073-14502-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>