drm/drm_internal.h: Remove repeated struct declaration

struct drm_gem_object is declared twice. One is declared
at 40th line. The blew one is not needed. Remove the duplicate.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210401081704.1000863-1-wanjiabing@vivo.com