drm/i915: simplify intel_ddi_pll_select()

Because {hsw,skl,bxt}_ddi_pll_select all pretty much do the same thing
in slightly different ways. Replace everything with a simple copy of
the function and inline it inside intle_ddi_pll_select().

v2: s/return pll/return pll != NULL/ (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1490209125-20046-1-git-send-email-paulo.r.zanoni@intel.com