kmsdrm: Remove gbm_bo_get_offset()

It is not present on some older libgbm versions that we still
support and has been unused since 85e8adf.

src/video/kmsdrm/SDL_kmsdrmsym.h

@@ -103,7 +103,6 @@ SDL_KMSDRM_SYM(unsigned int,gbm_bo_get_width,(struct gbm_bo *bo))
 SDL_KMSDRM_SYM(unsigned int,gbm_bo_get_height,(struct gbm_bo *bo))
 SDL_KMSDRM_SYM(uint32_t,gbm_bo_get_stride,(struct gbm_bo *bo))
 SDL_KMSDRM_SYM(uint32_t,gbm_bo_get_format,(struct gbm_bo *bo))
-SDL_KMSDRM_SYM(uint32_t,gbm_bo_get_offset,(struct gbm_bo *bo, int plane))
 SDL_KMSDRM_SYM(union gbm_bo_handle,gbm_bo_get_handle,(struct gbm_bo *bo))
 SDL_KMSDRM_SYM(int,gbm_bo_write,(struct gbm_bo *bo, const void *buf, size_t count))
 SDL_KMSDRM_SYM(struct gbm_device *,gbm_bo_get_device,(struct gbm_bo *bo))