drm/i915/fbc: rename the FBC disable functions

Instead of:
 - intel_fbc_disable_crtc(crtc)
 - intel_fbc_disable(dev_priv)
we now have:
 - intel_fbc_disable(crtc)
 - intel_fbc_global_disable(dev_priv)

This is because all the other functions that take a CRTC are called
 - intel_fbc_something(crtc)
Instead of:
 - intel_fbc_something_crtc(crtc)

And I also hope that the word "global" is going to help make it more
explicit that "global" is the unusual case, not the opposite.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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/1453210558-7875-14-git-send-email-paulo.r.zanoni@intel.com