drm/i915: allow platforms without eDP transcoder

Define a HAS_TRANSCODER_EDP() macro that checks if we have defined an
offset for this transcoder. This allows platforms to be defined without
eDP transcoder.

Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190222230254.20351-2-lucas.demarchi@intel.com