i915: Fix map/unmap mismatches from leaving INTEL_FALLBACK during TNL.

i915: Fix map/unmap mismatches from leaving INTEL_FALLBACK during TNL.

The first rendering after context create didn't know of the color
buffer yet, triggering a sw fallback.  The intel_prepare_render() from
intelSpanRenderStart then found the buffer and turned off fallbacks,
but intelSpanRenderFinish was never called and things were left
mapped.  By checking buffers before making the call on whether to do
the fallback pipeline or not, we avoid the fallback change inside of
the rendering pipeline.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31561
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>