drm/i915: Convert REG_GENMASK*() to fixed-width GENMASK_U*()

Now that include/linux/bits.h implements fixed-width GENMASK_U*(), use
them to implement the i915/xe specific macros. Converting each driver
to use the generic macros are left for later, when/if other
driver-specific macros are also generalized.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Yury Norov <yury.norov@gmail.com>