drm/ci: docs: add step about how to request privileges

Clarify the procedure developer must follow to request privileges to
run tests on Freedesktop gitlab CI.

This measure was added to avoid untrusted people to misuse the
infrastructure.

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Link: https://lore.kernel.org/r/20231024004525.169002-11-helen.koike@collabora.com


Signed-off-by: Maxime Ripard <mripard@kernel.org>