drm/i915: update FBC maximum fb sizes

CTG/ILK/SNB/IVB support 4kx2k surfaces. HSW supports 4kx4k, but
without proper front buffer invalidation on the last 2k lines, so
don't enable FBC on these cases for now.

v2: Use gen >= 5, not gen > 4 (Daniel).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>