radv: fix getting the index type size for uint8_t

16-bit and 32-bit values match hardware values but 8-bit doesn't.

This fixes dEQP-VK.pipeline.input_assembly.* with 8-bit index.

Fixes: 372c3dcf ("radv: implement VK_EXT_index_type_uint8")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 89671ef2)