intel: Don't spam "intelReadPixels: fallback to swrast" in non-PBO case.

intel: Don't spam "intelReadPixels: fallback to swrast" in non-PBO case.

When an application is using PBOs, we attempt to use the BLT engine to
perform ReadPixels.  If that fails due to some restrictions, it's useful
to raise a performance warning.

In the non-PBO case, we always use a CPU mapping since getting the data
into client memory requires a CPU-side copy.  This is a very common case,
so raising a performance warning is annoying.  In particular, apitrace's
image dumping code hits this path, causing it to print hundreds of
thousands of performance warnings via ARB_debug_output.  This tends to
obscure actual errors or other important messages.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>