drm/i915: Use intel_get_pipe_timings() and intel_mode_from_pipe_config() in intel_crtc_mode_get()

Eliminate the duplicate code for pipe timing readout in
intel_crtc_mode_get() by using the functions we use for the normal state
readout.

v2: Store dotclock in adjusted_mode instead of the final mode

Cc: dri-devel@lists.freedesktop.org
Cc: Rob Kramer <rob@solution-space.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1459536530-17754-1-git-send-email-ville.syrjala@linux.intel.com