drm/i915: Do not initialize display BW when display not available

When display is not available, finding the memory bandwidth available
for display is not useful. Skip this sequence here.

References: HSDES 1209978255

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120011016.18049-1-stuart.summers@intel.com