drm/i915: Treat SAGV block time 0 as SAGV disabled

For modern platforms the spec explicitly states that a
SAGV block time of zero means that SAGV is not supported.
Let's extend that to all platforms. Supposedly there should
be no systems where this isn't true, and it'll allow us to:
- use the same code regardless of older vs. newer platform
- wm latencies already treat 0 as disabled, so this fits well
  with other related code
- make it a bit more clear when SAGV is used vs. not
- avoid overflows from adding U32_MAX with a u16 wm0 latency value
  which could cause us to miscalculate the SAGV watermarks on tgl+

Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220309164948.10671-2-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
(cherry picked from commit d8f5855b

)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>