intel: Fix initial MakeCurrent for single-buffer drawables

intel: Fix initial MakeCurrent for single-buffer drawables

Commit 05da4a7a5e7d5bd988cb31f94ed8e1f053d9ee39 attempts to eliminate the
call to intel_update_renderbuffer() in the case where we already have a
drawbuffer for the drawable.  Unfortunately this only checks the
back left renderbuffer, which breaks in case of single buffer drawables.

This means that the initial viewport will not be set in that case.  Instead,
we now check whether the initial viewport has not been set, in which case
we call out to intel_update_renderbuffer().

https://bugs.freedesktop.org/show_bug.cgi?id=73862

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>