drm/i915: Move FBC init and cleanup calls to modeset functions

Although FBC helps save power it do not belongs to power management
also the cleanup was placed in i915_driver_unload() also not a good
place. intel_modeset_init()/intel_modeset_cleanup() are better places
also this will help make easy disable features that depends in
display being enabled in driver.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181108001647.11276-2-jose.souza@intel.com