anv: fix assert in anv_CmdBindDescriptorSets()

The assert is checking that we are not binding more descriptor sets
than the supported by the driver. When binding the descriptor set
number MAX_SETS-1, it was breaking the assert because
descriptorSetCount = 1.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit 0f290062)