drm/i915/uc: Fix undefined behavior due to shift overflowing the constant

Fix:

  In file included from <command-line>:0:0:
  drivers/gpu/drm/i915/gt/uc/intel_guc.c: In function ‘intel_guc_send_mmio’:
  ././include/linux/compiler_types.h:352:38: error: call to ‘__compiletime_assert_1047’ \
  declared with attribute error: FIELD_PREP: mask is not constant
    _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)

and other build errors due to shift overflowing values.

See https://lore.kernel.org/r/YkwQ6%2BtIH8GQpuct@zn.tnic for the gory
details as to why it triggers with older gccs only.

v2 by Jani:
- Drop the i915_reg.h changes

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Ruiqi GONG <gongruiqi1@huawei.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518113315.1305027-2-jani.nikula@intel.com