drm/i915: Add HAS_DISPLAY() and use it

Right now it is decided if GEN has display by checking the num_pipes,
so lets make it explicit and use a macro.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130232048.14216-1-jose.souza@intel.com