drm/i915/cnl: Simplify dco_fraction calculation.

I confess I never fully understood that previous calculation,
so this is not a "fix". But let's simplify this math
so poor brains like mine can read and make some sense of
it in the future.

v2: Don't follow the spec since that gives invalid
    values and it is also confusing. This Ville's
    version is much simpler.
v3: Use u64 cast instead of declaring a u64 dco. (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115184257.8633-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_dpll_mgr.c

@@ -2153,6 +2153,8 @@ static void cnl_wrpll_params_populate(struct skl_wrpll_params *params,
 				      u32 dco_freq, u32 ref_freq,
 				      int pdiv, int qdiv, int kdiv)
 {
+	u32 dco;
+
 	switch (kdiv) {
 	case 1:
 		params->kdiv = 1;
@@ -2189,9 +2191,10 @@ static void cnl_wrpll_params_populate(struct skl_wrpll_params *params,
 	params->qdiv_ratio = qdiv;
 	params->qdiv_mode = (qdiv == 1) ? 0 : 1;
 
-	params->dco_integer = div_u64(dco_freq, ref_freq);
-	params->dco_fraction = div_u64((div_u64((uint64_t)dco_freq<<15, (uint64_t)ref_freq) -
-					((uint64_t)params->dco_integer<<15)) * 0x8000, 0x8000);
+	dco = div_u64((u64)dco_freq << 15, ref_freq);
+
+	params->dco_integer = dco >> 15;
+	params->dco_fraction = dco & 0x7fff;
 }
 
 static bool