drm/i915: Use pipe A primary plane .max_stride() as the global stride limit

Let's assume that the primary plane for pipe A has the highest max
stride of all planes, and we'll use that as the global limit when
creating a new framebuffer.

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/20180907152413.15761-4-ville.syrjala@linux.intel.com