svga: change svga_destroy_shader_variant() to return void

svga: change svga_destroy_shader_variant() to return void

svga_destroy_shader_variant() itself flushes and retries the command
if there's a failure.  So no need for the callers to do it.  Other
callers of the function were already ignoring the return value.

This also fixes a corner-case double-free reported by Coverity
(and reported by Dave Airlie).

Tested with various OpenGL apps.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>