mesa/main: Expose EXT_sRGB_write_control

mesa/main: Expose EXT_sRGB_write_control

Use EXT_framebuffer_sRGB to expose EXT_sRGB_write_control on GLES. Remove
the checks for desktion GL in the enable calls, since EXT_framebuffer_sRGB
now also indicates support for switching the linear-sRGB color
space conversion on GLES.

Thanks to Ilia Mirkin for all the helpful discussions that helped to rework
this series.

v2: Fix alphabetical listing of extensions (Tapani Pälli)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)