mesa: Replace _mesa_unpack_bitmap with _mesa_unpack_image()

mesa: Replace _mesa_unpack_bitmap with _mesa_unpack_image()

_mesa_unpack_bitmap() was introduced by commit 02b801c to handle the case
when data is stored in PBO by display lists, in the context of this bug:

Incorrect pixels read back if draw bitmap texture through Display list
https://bugs.freedesktop.org/show_bug.cgi?id=10370

Since _mesa_unpack_image() already handles the case of GL_BITMAP, this patch
removes _mesa_unpack_bitmap() and makes affected calls go through
_mesa_unapck_image() instead.

The sample test attached to the original bug report passes with this change
and there are no piglit regressions.

Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>