drm/i915/fbc: Loop through FBC instances in various places

Convert i915->fbc into an array in preparation for
multiple FBC instances, and loop through all instances
in all places where the caller does not know which
instance(s) (if any) are relevant. This is the case
for eg. frontbuffer tracking and FIFO underrun hadling.

v2: More intel_ namespace (Jani)
    Leave out debugfs for later

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211213134450.3082-3-ville.syrjala@linux.intel.com