drm/msm/gen_header: allow skipping the validation

We don't need to run the validation of the XML files if we are just
compiling the kernel. Skip the validation unless the user enables
corresponding Kconfig option. This removes a warning from gen_header.py
about lxml being not installed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20240409120108.2303d0bd@canb.auug.org.au/
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/592558/
Signed-off-by: Rob Clark <robdclark@chromium.org>