drm/selftests/test-drm_framebuffer: Remove set but unused variable 'fb'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/selftests/test-drm_framebuffer.c: In function ‘execute_drm_mode_fb_cmd2’:
 drivers/gpu/drm/selftests/test-drm_framebuffer.c:333:26: warning: variable ‘fb’ set but not used [-Wunused-but-set-variable]

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-29-lee.jones@linaro.org