pvr: Fix seg fault in vkAllocateDescriptorSets().

In cases when no immutable samplers were present but sampler
descriptor set layout bindings were, a seg fault was being caused
by an attempt to get the immutable sampler from within the
immutable sampler array while the array was not allocated.

This commit also remove the binding type check since only those
specific types can have immutable samplers. The check is covered
by the descriptor set layout creation and assignment of
has_immutable_samplers.

This commit also makes the immutable samplers const, since they're
meant to be immutable.

This commit also adds has_immutable_samplers field to descriptor
set layout. Previously to check whether immutable
samplers were present or not you'd check for the layout binding's
descriptor count to be 0 and the immutable sampler offset to be 0.
This doesn't tell you everything. If you have a descriptor of the
appropriate type (VK_DESCRIPTOR_TYPE_{COMBINED_,}IMAGE_SAMPLER).
So descriptor count of >1. The offset can be 0 if you don't have
immutable sampler, or have them at the beginning of the immutable
samplers array. So you can't determine if you really have immutable
samplers or not. One could attempt to perform a NULL check on the
array but this would not work in cases where you have following set
layout bindings with immutable samplers as the array stores the
whole layout's immutable samplers.

Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Rajnesh Kanwal <rajnesh.kanwal@imgtec.com>
Part-of: <!15688>