drm/imagination: annotate pvr_fw_version_packed() with __maybe_unused

drm/imagination: annotate pvr_fw_version_packed() with __maybe_unused

Building with clang and W=1 leads to warning about unused
pvr_fw_version_packed(). Fix by annotating it with __maybe_unused.

See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
Link: https://patchwork.freedesktop.org/patch/msgid/190e4eefef6c5e62052a01af0084c69361e216ef.1725962479.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>