drm/gma500/mmu: Make local function 'psb_mmu_pt_alloc_map_lock()' static

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/gma500/mmu.c:316:20: warning: no previous prototype for ‘psb_mmu_pt_alloc_map_lock’ [-Wmissing-prototypes]

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-19-lee.jones@linaro.org