drivers: gpu: drm: msm: registers: improve reproducibility

The files generated by gen_header.py capture the source path to the
input files and the date.  While that can be informative, it varies
based on where and when the kernel was built as the full path is
captured.

Since all of the files that this tool is run on is under the drivers
directory, this modifies the application to strip all of the path before
drivers.  Additionally it prints <stripped> instead of the date.

Both changes solve the reproducibility issue.

Upstream-Status: Inappropriate

Signed-off-by: Ryan Eatmon <reatmon@ti.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>