mesa/extensions: Create _mesa_extension_supported()

mesa/extensions: Create _mesa_extension_supported()

Create a function which determines if an extension is supported in the
current context.

v2: Use common variable names (Emil)
    Insert new line between variables and return statement (Chad)
    Rename api_set variable to api_bit (Chad)

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>