mesa: Use a single enable for GL_EXT_texture_array and GL_MESA_texture_array

mesa: Use a single enable for GL_EXT_texture_array and GL_MESA_texture_array

Every driver that enables one also enables the other.  The difference
between the two is MESA adds support for fixed-function and assembly
fragment shaders, but EXT only adds support for GLSL.  The MESA
extension was created back when Mesa did not support GLSL.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>