i965: Gut the separate OpenGL ES extension enabling.

i965: Gut the separate OpenGL ES extension enabling.

We should just set the bits of functionality that we support; the
GL/ES1/ES2 flags in extensions.c will take care of advertising the
appropriate extensions for the current API.

This enables the GL_EXT_texture_compression_dxt1 extension on ES1/ES2
when libtxc_dxtn is installed or the force_s3tc driconf option is set.
The main extension code set this up properly, but the ES-specific code
failed to do so.

Otherwise, the extension strings reported by es1_info, es2_info, and
glxinfo all remain the same.

This patch manually disables the ARB_framebuffer_object bit on ES
to preserve the behavior of 1c0f5d8324c4db2720247989ddc4a45315b55a85.

v2: Rebase, fix the i915 Makefile, and unconditionally set the
    OES_draw_texture bit as core Mesa will only apply it to ES1 now.

Tested-by: Daniel Charles <daniel.charles@intel.com> [v1]
Reviewed-by: Chad Versace <chad.versace@linux.intel.com> [v1]
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>