drm: samsung: fix implicit fall-through errors

Fixes several -Wimplicit-fallthrough errors similar to:

  display/samsung/exynos_drm_debug.c:109:2:
  error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case DPU_EVT_DECON_FRAMEDONE:
        ^
  display/samsung/exynos_drm_debug.c:109:2:
  note: insert '__attribute__((fallthrough));' to silence this warning
        case DPU_EVT_DECON_FRAMEDONE:
        ^
        __attribute__((fallthrough));
  display/samsung/exynos_drm_debug.c:109:2:
  note: insert 'break;' to avoid fall-through
        case DPU_EVT_DECON_FRAMEDONE:
        ^
        break;

Bug: 215617713
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I5f8976441ea0031efbef0c546ee3f3181886af20