drm/i915/cnl: don't hardcode DPCLKA_CFGCR0_DDI_CLK_SEL_SHIFT

We have the macro, use it. Makes the code a little easier to
understand.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170825194004.26571-1-paulo.r.zanoni@intel.com