intel: Remove num_mapped_regions assertion from _intel_batchbuffer_flush

intel: Remove num_mapped_regions assertion from _intel_batchbuffer_flush

There are cases where a buffer can be mapped while another buffer is
flushed.  This can happen in the CopyPixels meta-op path for piglit's
fbo-mipmap-copypix.  After some discussion with Eric, it seems this
assertion is no longer necessary, and it has always been too strict.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43328
Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>