egl/x11: calloc dri2_surf so it's properly zeroed

Commit 2282ec0a refactored drawable creation across various platforms
into a new dri2_create_drawable helper function.

The GBM code in platform_drm.c code passed in dri2_surf->gbm_surf as the
loaderPrivate, while most other backends passed in dri2_surf directly.

To try and handle this, the patch checked if dri2_surf->gbm_surf was
non-NULL, and if so, presumed that the caller is the DRM platform and
we should use the dri2_surf->gbm_surf pointer.

This worked for most platforms, which calloc their dri2_surf structure,
zeroing the data.  Unfortunately, platform_x11.c used malloc, leaving
most of the dri2_surf as garbage.  In particular, dri2_surf->gbm_surf
was often non-NULL, causing dri2_create_drawable to try and use it,
passing a garbage pointer to the createNewDrawable hook, usually leading
to a SIGBUS or SIGSEGV when trying to dereference that bad pointer.

Since most callers calloc the data, make platform_x11.c follow suit.

Fixes crashes with i915_dri.so when running dEQP-GLES2.

Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 4e3297f7)