drm/i915: Fix clang -Wimplicit-fallthrough in intel_async_flip_check_hw()

drm/i915: Fix clang -Wimplicit-fallthrough in intel_async_flip_check_hw()

Clang warns:

  drivers/gpu/drm/i915/display/intel_display.c:6012:3: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
                  case I915_FORMAT_MOD_X_TILED:
                  ^
  drivers/gpu/drm/i915/display/intel_display.c:6012:3: note: insert 'break;' to avoid fall-through
                  case I915_FORMAT_MOD_X_TILED:
                  ^
                  break;
  1 error generated.

Clang is a little more pedantic than GCC, which does not warn when
falling through to a case that is just break or return. Clang's version
is more in line with the kernel's own stance in deprecated.rst, which
states that all switch/case blocks must end in either break,
fallthrough, continue, goto, or return. Add the missing break to silence
the warning.

Fixes: 937859485aef ("drm/i915: Support Async Flip on Linear buffers")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/202305241902.UvHtMoxa-lkp@intel.com/
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/CA+G9fYv68V3ewK0Qj-syQj7qX-hQr0H1MFL=QFNuDoE_J2Zu-g@mail.gmail.com/
Reported-by: Tom Rix <trix@redhat.com>
Closes: https://lore.kernel.org/all/20230523125116.1669057-1-trix@redhat.com/
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230524-intel_async_flip_check_hw-implicit-fallthrough-v1-1-83de89e376a1@kernel.org