egl/dri2: NULL check value returned by dri2_create_surface

egl/dri2: NULL check value returned by dri2_create_surface

dri2_create_surface can fail for a variety of reasons, including bad
input data.  Dereferencing the NULL pointer and crashing is not okay.

Fixes issue identified by Klocwork analysis:

    Pointer 'surf' returned from call to function 'dri2_create_surface'
    at line 285 may be NULL and will be dereferenced at line 291.

NOTE: This is a candidate for the stable branches.

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