drm/i915/mtl: avoid stringop-overflow warning

drm/i915/mtl: avoid stringop-overflow warning

The newly added memset() causes a warning for some reason I could not
figure out:

In file included from arch/x86/include/asm/string.h:3,
                 from drivers/gpu/drm/i915/gt/intel_rc6.c:6:
In function 'rc6_res_reg_init',
    inlined from 'intel_rc6_init' at drivers/gpu/drm/i915/gt/intel_rc6.c:610:2:
arch/x86/include/asm/string_32.h:195:29: error: '__builtin_memset' writing 16 bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=]
  195 | #define memset(s, c, count) __builtin_memset(s, c, count)
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/i915/gt/intel_rc6.c:584:9: note: in expansion of macro 'memset'
  584 |         memset(rc6->res_reg, INVALID_MMIO_REG.reg, sizeof(rc6->res_reg));
      |         ^~~~~~
In function 'intel_rc6_init':

Change it to an normal initializer and an added memcpy() that does not have
this problem.

Fixes: 4bb9ca7ee074 ("drm/i915/mtl: C6 residency and C state type for MTL SAMedia")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231016201012.1022812-1-arnd@kernel.org