drm/i915: Add .max_stride() plane hook

Each plane may have different stride limitations. Let's add a new
plane function to retutn the maximum stride for each plane. There's
going to be some use for this outside the .atomic_check() stuff hence
the separate hook.

v2: Fix ilk+ x-tiled max stride to be 32k (José)

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907152413.15761-3-ville.syrjala@linux.intel.com