drm/i915: Simplify LVDS crtc_mask setup

drm/i915: Simplify LVDS crtc_mask setup

We don't need to special case PCH vs. gen4 when setting up the LVDS
crtc_mask. Just claim pipes A|B|C work and
intel_encoder_possible_crtcs() will drop out any crtc that doesn't
exist.

v2: Put the special case first to match what most other encoders do

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002162505.30716-2-ville.syrjala@linux.intel.com
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>