drm/i915/fbc: sanitize i915.enable_fbc during FBC init

The DDX driver changes its behavior depending on the value it reads
from i915.enable_fbc, so sanitize the value in order to allow it to
know what's going on. It uses this in order to choose the defaults for
the TearFree option. Before this patch, it would read -1 and always
assume that FBC was disabled, so it wouldn't force TearFree.

v2: Extract intel_sanitize_fbc_option() (Chris).
v3: Rebase.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460574069-14005-1-git-send-email-paulo.r.zanoni@intel.com