mesa/main: fix validation of GL_TIMESTAMP

mesa/main: fix validation of GL_TIMESTAMP

ctx->Extensions.ARB_timer_query is set based on the driver-
capabilities, not based on the context type. We need to check
against _mesa_has_ARB_timer_query(ctx) instead to figure out
if the extension is really supported. We also need to check for
EXT_disjoint_timer_query for GLES-support.

This shouln't have any functional effect, as this entry-point is only
valid on desktop GL, or on GLES with EXT_disjoint_timer_query in the
first place. But if this gets added to the core of a future version
of ES, this should be a step in the right direction.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>