drm/radeon/kms: fix up for BIG ENDIAN breakage

Commit 308e5bcb ("drm: add an fb creation ioctl that takes a pixel
format v5") missed one spot needing to be fixed up in the __BIG_ENDIAN
case.

Fixes build error:

drivers/gpu/drm/radeon/radeon_fb.c: In function
'radeonfb_create_pinned_object':
drivers/gpu/drm/radeon/radeon_fb.c:144:18: error: 'struct drm_mode_fb_cmd2'
has no member named 'bpp'

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dave Airlie <airlied@redhat.com>