drm/i915: set TRANSCODER_EDP even earlier

Instead of setting it at the beginning of haswell_crtc_mode_set, let's
set it at the beginning of intel_crtc_mode_set. When
intel_crt_mode_set calls drm_vblank_pre_modeset we already need to
have the transcoder_edp correctly set, because eventually
drm_vblank_pre_modeset calls functions that call i915_pipe_enabled
from i915_irq.c, which will read PIPECONF(cpu_transcoder).

This is a bug that affects us since we added support for
TRANSCODER_EDP, but I was only able to see the problem after
suspending a machine with the power well disabled (got an "unclaimed
register" error.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>