intel: Allow blit readpixels even when the pack alignment is set.

intel: Allow blit readpixels even when the pack alignment is set.

The default alignment is 4, so this fast path was rarely hit.  Rather
than introduce logic to handle alignment, just use the Mesa core
function.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=46632
Cc: neil@linux.intel.com
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>