gpu: host1x: Fix build warnings

When debugfs support isn't enabled, gcc complains about some variables
being unused. To avoid further #ifdefery, move debugfs specific setup
code into static functions and use IS_ENABLED(CONFIG_DEBUG_FS) to have
the compiler, rather than the preprocessor, discard them when unused.
The advantage of doing it this way is that all the code will be
compile-tested whether or not debugfs support is enabled.

Signed-off-by: Thierry Reding <treding@nvidia.com>