asahi: Do not overread user index buffers

We need to align the extent to 4 for the GPU, but we can't copy more
than the original size out of the user buffer. Always alloc the exact
size we need.

This does mean the GPU gets an IB extent that could include some other
stuff later in the pool, if not aligned. This is probably safe? Given
the base alignment, it should never cross a page boundary and fault or
anything like that.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21686>

src/gallium/drivers/asahi/agx_state.c

@@ -2389,8 +2389,9 @@ agx_index_buffer_direct_ptr(struct agx_batch *batch,
    } else {
       *extent = ALIGN_POT(draw->count * info->index_size, 4);
 
-      return agx_pool_upload_aligned(
-         &batch->pool, ((uint8_t *)info->index.user) + offset, *extent, 64);
+      return agx_pool_upload_aligned(&batch->pool,
+                                     ((uint8_t *)info->index.user) + offset,
+                                     draw->count * info->index_size, 64);
    }
 }