drm/sti: Fix compilation failure for drm_framebuffer.pixel_format

drivers/gpu/drm/sti/sti_plane.c:76:33: error: ‘struct drm_framebuffer’
has no member named ‘pixel_format’; did you mean ‘format’?

I didn't look to hard at the casting to a char * and just did a
mechanical transformation of s/pixel_format/format->format/ as given in
commit 438b74a5 ("drm: Nuke fb->pixel_format").

Fixes: 438b74a5 ("drm: Nuke fb->pixel_format")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>