Add support for GENERIC modifiers

[emersion]

Latest drm_fourcc.h has:

#define DRM_FORMAT_MOD_GENERIC_16_16_TILE DRM_FORMAT_MOD_SAMSUNG_16_16_TILE

We can't just include GENERIC in our list of vendors, because that fails with:

tables.c: In function ‘basic_modifier_str’:
tables.c:249:2: error: duplicate case value
  249 |  case DRM_FORMAT_MOD_SAMSUNG_16_16_TILE:
      |  ^~~~
tables.c:241:2: note: previously used here
  241 |  case DRM_FORMAT_MOD_GENERIC_16_16_TILE:
      |  ^~~~