drm/i915/cnp: Get/set proper Raw clock frequency on CNP.

RAWCLK_FREQ register has changed for platforms with CNP+.

[29:26] This field provides the denominator for the fractional
	part of the microsecond counter divider.  The numerator
	is fixed at 1. Program this field to the denominator of
	the fractional portion of reference frequency minus one.
	If the fraction is 0, program to 0.
	0100b = Fraction .2 MHz = Fraction 1/5.
	0000b = Fraction .0 MHz.

[25:16] This field provides the integer part of the microsecond
	counter divider. Program this field to the integer portion
	of the reference frequenct minus one.

Also this register tells us that proper raw clock should be read
from SFUSE_STRAP and programmed to this register. Up to this point
on other platforms we are reading instead of programming it so
probably relying on whatever BIOS had configured here.

Now on let's follow the spec and also program this register
fetching the right value from SFUSE_STRAP as Spec tells us to do.

v2: Read from SFUSE_STRAP and Program RAWCLK_FREQ instead of
    reading the value relying someone else will program that
    for us.
v3: Add missing else. (Jani)
v4: Addressing all Ville's catches:
    Use macro for shift bits instead of defining shift.
    Remove shift from the cleaning bits with mask that already
    has it.
    Add missing I915_WRITE to actually write the reg.
    Stop using useless DIV_ROUND_* on divider that is exact
    dividion and use DIV_ROUND_CLOSEST for the fraction part.
v5: Remove useless Read-Modify-Write on raclk_freq reg. (Ville).
v6: Change is per PCH instead of per platform.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496434004-29812-3-git-send-email-rodrigo.vivi@intel.com