gpu: host1x: Make host1x_cdma_wait_pushbuffer_space() static

The host1x_cdma_wait_pushbuffer_space() function is not declared or
directly called from outside the file it is in, so make it static.

Fixes the following sparse warning:

    drivers/gpu/host1x/cdma.c:235:5: warning: symbol 'host1x_cdma_wait_pushbuffer_space' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Thierry Reding <treding@nvidia.com>