drivers: gpu: Mark functions as static in vmwgfx_buffer.c

Mark functions as static because they are not used outside the file
drm/vmwgfx/vmwgfx_buffer.c.

This eliminates the following warnings in drm/vmwgfx/vmwgfx_buffer.c:
drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c:520:16: warning: no previous prototype for ‘vmw_ttm_tt_create’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c:549:5: warning: no previous prototype for ‘vmw_invalidate_caches’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c:554:5: warning: no previous prototype for ‘vmw_init_mem_type’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c:592:6: warning: no previous prototype for ‘vmw_evict_flags’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>