mesa/dri: Move context flag validation down into the drivers

mesa/dri: Move context flag validation down into the drivers

Soon some drivers will support a different set of flags than other
drivers.  If some flags have to be filtered in the driver, we might as
well filter all of them in the driver.

The changes in nouveau use tabs because nouveau seems to have it's own
indentation rules.

v2: Fix some rebase failures noticed by Ken (returning the wrong types,
etc.).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>