drm/i915/blt: bump the size restriction

drm/i915/blt: bump the size restriction

As pointed out by Chris, with our current approach we are actually
limited to S16_MAX * PAGE_SIZE for our size when using the blt to clear
pages. Keeping things simple try to fix this by reducing the copy to a
sequence of S16_MAX * PAGE_SIZE blocks.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[ickle: hide the details of the engine pool inside emit_vma]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190810092945.2762-1-chris@chris-wilson.co.uk