drm/crtc-helper: use drm_framebuffer flags

The "flags" parameter of the DRM_IOCTL_MODE_ADDFB2 ioctl must be
propagated and used by the driver.
The only possible value of flags is DRM_MODE_FB_INTERLACED.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Reviewed-by: Benjamin GAIGNARD <benjamin.gaignard@st.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>