drm/i915: make intel_ddi_get_cdclk_freq return values in KHz

drm/i915: make intel_ddi_get_cdclk_freq return values in KHz

With this, that 338 can finally become the correct 337500.

Due to the change we need to adjust the intel_dp_aux_ch function to
set the correct value, so adjust the division and also use
DIV_ROUND_CLOSEST instead of the old "round down" behavior because the
spec says the value "should be programmed to get as close as possible
to the ideal rate of 2MHz".

Quoting Paulo's follow-up to a question from Chris Wilson to explain
what exactly will change:

I use the 337500 value on the next patch, when setting the
ips_linetime value. The correct frequency is 337500, not 338000.

ips_linetime = DIV_ROUND_CLOSEST(mode->htotal * 1000 * 8,
intel_ddi_get_cdclk_freq);
For a mode with htotal of 2640 [0] we'll have: (i) (2640 * 1000 * 8) /
338000 = 62.48, resulting in 62 and (ii) (2640 * 1000 * 8) / 337500 =
62.57 resulting in 63.

For the case inside intel_dp.c:
Previously we were using 338. So with the old formula we were writing
338/2 = 169 to the register. And 337500 / 169 = 1997.04 (we use 337500
here because it's the real clock value). With the new value of
337500/2000 we'll have 168.75, which is 168 on the round-down case and
169 on the round-closest case. If we write 168 to the register, 337500
/ 168 = 2008.92, and 2008.92 is more distant from 2000 than 1997.04.
So with this patch we're changing the formula but still writing the
same correct value to the DP AUX register.

[0]: That's 1920x1080@50Hz on my DP monitor.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Pimp the commit message with Paulo's follow-up.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>