egl: Add support for driconf control of swapinterval.

egl: Add support for driconf control of swapinterval.

This behavior mostly matches glx_dri2.  It's slightly complicated in
comparison because EGL exposes the implementation limits in the EGL config.

Note that platform_x11 was the only one setting swap_available, so the move of
the MaxSwapInterval into it is appropriate.

Acked-by: Chad Versace <chad.versace@linux.intel.com>