drm/i915/skl: Fix always true comparison in a revision id check

It's always a good idea to keep static analysis happy (also because it
prompts doing the check like I proposed :), this time smatch complains:

drivers/gpu/drm/i915/intel_ringbuffer.c:891 gen9_init_workarounds() warn:
  always true condition '((->dev->pdev->revision) >= (0)) => (0-255 >= 0)'

That's because revision is a u8. Tweak a bit the condition then.

Cc: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>