drm/i915: Split skl_get_dpll()

Split out the DisplayPort and HDMI pll setup code into separate
functions and refactor the DP code does not directly depend on
crtc state, so that the code can be used for upfront link training.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>