Don't allow FBC for cases where the spec says we can't FBC.

v2:
  - Just WARN_ON() the strides that should have been caught earlier
    (Daniel)
  - Make it a new function since I expect this to grow more.
v3:
  - Document which IGT test is exercised by this.
v4:
  - Implement the restrictions for gens 2-6 too (Ville).
  - Fix off-by-one mistake (Ville).

Testcase: igt/kms_frontbuffer_tracking/fbc-badstride
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>