i965: Move intel_prepare_render() above first buffer access

i965: Move intel_prepare_render() above first buffer access

The driver is supposed to ensure buffers before any drawing operation, but in
do_blit_drawpixels() and do_blit_copypixels() we inspect the buffer format
before calling intel_prepare_render().  That was covered up by the
unconditional call to intel_prepare_render() in intelMakeCurrent(), but we
now only do this on the initial intelMakeCurrent call for a context
(to get the size for the initial viewport values).

https://bugs.freedesktop.org/show_bug.cgi?id=74083

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Tested-by: Alexander Monakov <amonakov@gmail.com>