drm/i915: Let's use more enum intel_dpll_id pll_id.

No functional change expected. Just let's use this enum
when possible and also same standard pll_id name
so we can rework gen9+ port clock later.

Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171018195407.8618-1-rodrigo.vivi@intel.com