drm/i915: Make i9xx_crtc_clock_get() use dpll_hw_state

We already extract the DPLL state to pipe_config, so let's make use of
it in i9xx_crtc_clock_get() and avoid the register reads.

This will also make the function closer to being useable with PCH DPLL
since the registers for those live in a different address.

Also kill the useless adjusted_mode.clock zeroing. It's already zero at
this point.

v2: Read out DPLL state in intel_crtc_mode_get()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>