drm/i915: don't recheck for invalid pipe bpp

As noticed by Daniel Vetter, intel_pipe_choose_bpp_dither should
already check for invalid bpp values and set a valid value, so remove
the recheck inside ironlake_crtc_mode_set and also replace a "default"
switch case inside ironlake_set_pipeconf with a BUG().

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>