Decoding the register can be only as good as the reg_spec being used.
The current builtin register spec is not that. Move that decoding behind
an option to make it better for the normal case when running from the
repo checkout where the external reg spec is not available. Passing any
of --decode, --all, --spec brings the old behavior back:

	$ sudo ./build/tools/intel_reg --decode read 0x2358
	Warning: stat '/usr/local/share/igt-gpu-tools/registers' failed: No such file or directory. Using builtin register spec.
					    (0x00002358): 0x00000000

vs the new behavior:

	$ sudo ./build/tools/intel_reg --decode read 0x2358
					    (0x00002358): 0x00000000

We could probably reduce the leading space since we won't have any name,
but that can be improved later.

v2: Instead of removing the warning, move the whole decoding logic
    behind a command line option
v3: Some commands also imply --decode

Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Link: https://lore.kernel.org/r/20240426153352.3209972-2-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>