drm/nouveau/fb: Prevent inlining of ramfuc_reg

When gcc 4.8 inlines this function, it eats up 16 bytes on the stack
every time. Eventually we hit warnings because our stack grew too
much:

ramnve0.c:1383:1: error: the frame size of 1496 bytes is larger than
1024 bytes

We fix this by preventing inlining for this function.

Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>