radeonsi: support for external buffers (ext_external_objects)

radeonsi: support for external buffers (ext_external_objects)

So far, the callback to create a resource from a memory object had code
for importing textures only. Modified it to allow importing buffers too.

Fixes the following piglit tests:
- ext_external_objects/vk-buf-exchange
- ext_external_objects/vk-pix-buf-update-errors
- ext_external_objects/vk-vert-buf-update-errors
- ext_external_objects/vk-vert-buf-reuse

v2: Used si_alloc_buffer_struct instead of CALLOC
v3: Fixed indentation issue, removed free in case of unsuccessful
allocation, joined two if conditions together

Signed-off-by: Eleni Maria Stea <estea@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6364>