mesa/main: do not allow EXT_texture_sRGB_R8 enums before gles3

mesa/main: do not allow EXT_texture_sRGB_R8 enums before gles3

ctx->Extensions.EXT_texture_sRGB_R8 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: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>