drm/fb_helper: Fix potential NULL pointer dereference

kcalloc returns NULL on failure. Hence check for the return value
and exit on error to avoid NULL pointer dereference.

Fixes the following smatch errors:
drivers/gpu/drm/drm_fb_helper.c:1271 drm_setup_crtcs() error:
potential null dereference 'modes'.  (kcalloc returns null)
drivers/gpu/drm/drm_fb_helper.c:1272 drm_setup_crtcs() error:
potential null dereference 'crtcs'.  (kcalloc returns null)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>