drm/i915: Avoid BIT(max) - 1 and use GENMASK(max - 1, 0)

drm/i915: Avoid BIT(max) - 1 and use GENMASK(max - 1, 0)

"BIT(max) - 1" will overflow when max = 32, and GCC will complain.
We already have GENMASK for generating the mask, use it!

v2: Majestic off by one spotted (Chris)

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>