drm/i915: Remove PCH type checks from ironlake_crtc_compute_clock()

The checks were added in commit 5dc5298b ("drm/i915: add proper
CPU/PCH checks to crtc_mode_set functions") in a time when there was
doubts on what PCHs would be supported by HSW. There are similar checks
for PCH type in intel_detect_pch() and the function pointers are
initialized based on platform/pch information, so the removed WARN can't
ever be reached.

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/1458576016-30348-8-git-send-email-ander.conselvan.de.oliveira@intel.com