drm/i915/fbc: make FBC work with fastboot

Move intel_fbc_enable to a place where it is called regardless of the
"modeset" variable, and make sure intel_fbc_enable can be called
multiple times without intel_fbc_disable being called.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-20-git-send-email-paulo.r.zanoni@intel.com