drm/i915/fbc: fix the FBC kernel-doc warnings

Fix the recently introduced 'make htmldocs' warnings:

$ make htmldocs 2>&1 > /dev/null | grep i915
./drivers/gpu/drm/i915/display/intel_fbc.c:635: warning: Excess function parameter 'i915' description in 'intel_fbc_is_active'
./drivers/gpu/drm/i915/display/intel_fbc.c:1638: warning: Excess function parameter 'i915' description in 'intel_fbc_handle_fifo_underrun_irq'
./drivers/gpu/drm/i915/display/intel_fbc.c:635: warning: Function parameter or member 'fbc' not described in 'intel_fbc_is_active'
./drivers/gpu/drm/i915/display/intel_fbc.c:635: warning: Excess function parameter 'i915' description in 'intel_fbc_is_active'
./drivers/gpu/drm/i915/display/intel_fbc.c:1638: warning: Function parameter or member 'fbc' not described in 'intel_fbc_handle_fifo_underrun_irq'
./drivers/gpu/drm/i915/display/intel_fbc.c:1638: warning: Excess function parameter 'i915' description in 'intel_fbc_handle_fifo_underrun_irq'

Fixes: e49a656b ("drm/i915/fbc: Start passing around intel_fbc")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211115140549.27629-1-jani.nikula@intel.com