drm/i915/skl: Adjust the port PLL selection code

Skylake deprecates the usage of PORT_CLK_SEL and we are advised to use
the new DPLL_CRTL2 for the DDI->PLL mapping.

v2: Modified as per review comments

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>