drm/i915: Check pipe_config.has_dp_encoder instead of encoder types

More concise. Noticed while reviewing Ander's patch which touched a
lot of the pipe_has_type checks.

v2: Use new_config in one place Ander spotted.

Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>