RELEASING: Fix releasing instructions to match the latest release.sh

It seems that the tests don't need DRM master anymore? at least make
distcheck passes when X is running.

release.sh is also invoked with just the path to the libdrm git checkout
and we don't want to pass additional arguments that will be treated as
additional modules we want to release.

Also, make a note that release.sh will run make distcheck for you, so we
don't strickly need to run it beforehand.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>