drm/i915/bxt: Modified HAS_CSR, added support for BXT

drm/i915/bxt: Modified HAS_CSR, added support for BXT

Modified HAS_CSR macro defination which earlier only supported
for skl, now added support for BXT.

v1: Initial version.

v2: Instaed of skylake/broxton check added gen9 check alone based
on review comment from Sunil.

Cc: Vetter, Daniel <daniel.vetter@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>