svga: fix max number of texture cube levels
The maximum number of mipmap levels supported for cubemap can be determined from the maximum 2D texture size. There is no need to limit the max to 12.
This fixes a regression in creating GL4.1 and up context since commit 2658d025 is now explicitly checking for MaxCubeTextureLevels >= 15 for GL4.1 context.
Reviewed-by: Neha Bhende bhenden@vmware.com
/cc @bhenden