mesa/main: do not allow sRGB texture enums before gles3

mesa/main: do not allow sRGB texture enums before gles3

ctx->Extensions.EXT_texture_sRGB is set regardless of the API that's
used, so checking for those direcly will always allow the enums from
this extensions when they are supported by the driver.

There's no extension adding support for this on OpenGL ES before
version 3.0, so let's tighten the check.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>