drm/i915: use proper FBC base register on all new platforms

Starting from GEN5 the FBC base register is the same on all platforms.
GEN>=5 is the same condition as HAS_PCH_SPLIT except on BXT, so make
things work on BXT as well.

Motivated by Rodrigo's request to check FBC support on BXT.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>