drm/armada: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/armada/armada_gem.c:215:27: warning: no previous prototype for 'armada_gem_alloc_object' [-Wmissing-prototypes]
drivers/gpu/drm/armada/armada_gem.c:423:1: warning: no previous prototype for 'armada_gem_prime_map_dma_buf' [-Wmissing-prototypes]

In fact, both functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>