drm/i915: Fix DPLCLKA_CFGCR0 bits for Port F.

Since when it got introduced with commit '555e38d2
("drm/i915/cnl: DDI - PLL mapping")' the support for Port F
was wrong, because Port F bits are far from bits used
for A to E.

Since Port F is not used so far we don't need to propagate
Fixes back there.

v2: Reuse _SHIFT definition to avoid complicated duplication (DK).

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180129232223.766-5-rodrigo.vivi@intel.com