dri_util: Use calloc to allocate __DRIcontext

dri_util: Use calloc to allocate __DRIcontext

The __DRIcontext contains some pointers, and some drivers check for them to be
NULL in some failure paths.  Instead of sprinkling NULL assignments across the
various drivers, just zero out the whole thing.

NOTE: This is a candidate for the 9.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-and-tested-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Lu Hua <huax.lu@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=53618
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54301
(cherry picked from commit f93cb0bebb10e3e3e5df099be51021b211650356)