mesa/main: do not allow EXT_texture_shared_exponent enums before gles3

mesa/main: do not allow EXT_texture_shared_exponent enums before gles3

ctx->Extensions.EXT_texture_shared_exponent 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.

We also need to make sure this is enabled on OpenGL ES 3. Because the
check is repeated, let's introduce a helper.

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