drm/i915: Kill DRAIN_LATENCY_PRECISION_* defines

Kill the silly DRAIN_LATENCY_PRECISION_* defines and just use the raw
number instead.

v2: Move the sprite 32/16 -> 16/8 preision multiplier
    change to another patch (Jesse)

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>