drm/i915: Split hsw_get_dpll()

Split out the DisplayPort and HDMI pll setup code into separate
functions and refactor the DP code that calculates the pll
so that it doesn't depend on crtc state.
This will be used for acquiring port pll when doing
upfront link training.

Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>