drm/i915/chv: find the best divisor for the target clock v4

Based on the chv clock limit, find the best divisor.

The divisor data has been verified with this spreadsheet.
P1273_DPLL_Programming Spreadsheet.

v2: Rebase the code and change the chv_find_best_dpll based on new
standard way to use intel_PLL_is_valid.  Besides, clean up some extra
variables.

v3: Ville suggest better fixed point for m2 calculation.

v4: -Add comment for the limit is compute using fast clock. (Ville)
	-Don't pass the request clock to chv_clock, as the same function will
	 be use clock readout, which doens't have request clock. (Ville)
	-Add and use DIV_ROUND_CLOSEST_ULL to consistent with other clock
	calculation. (Ville)
	-Fix the dp m2 after m2 has stored fixed point. (Ville)

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
[vsyrjala: Avoid div-by-zero in chv_clock()]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>