drm/i915: Simplify ironlake_crtc_compute_clock() CPU eDP case

None of the code in ironlake_crtc_compute_clock() is relevant for CPU
eDP. The CPU eDP PLL is turned on and off in ironlake_edp_pll_{on,off}
from the DP code and that doesn't depend on the crtc_state->dpll values,
so just return early in that case.

v2: Rebase without patch that drops lvds downclock code. (Ville)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-9-git-send-email-ander.conselvan.de.oliveira@intel.com