drm/i915: Fix declaration of intel_gmbus_{is_forced_bit/is_port_falid}

drm/i915: Fix declaration of intel_gmbus_{is_forced_bit/is_port_falid}

Description:
intel_gmbus_is_forced_bit is no extern as its body is right below.
Likewise for intel_gmbus_is_port_valid.

This fixes a compilation issue with clang. An initial version of this patch
was developed by PaX Team <pageexec at freemail.hu>.
This is respin of this patch.

20130509: v2: (re-)add inline upon request.

Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
CC: pageexec@freemail.hu
CC: daniel.vetter@ffwll.ch
CC: airlied@linux.ie
CC: intel-gfx@lists.freedesktop.org
CC: dri-devel@lists.freedesktop.org
CC: linux-kernel@vger.kernel.org
[danvet: Bikeshed commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>