This change prepares for VK_ANDROID_native_buffer. When the user imports
a gralloc hande into a VkImage using VK_ANDROID_native_buffer, the user
provides no size. The driver must infer the size from the internals of
the gralloc buffer.

The patch is essentially a refactor patch, but it does change behavior
in some edge cases, described below. In what follows, the "nominal size"
of the bo refers to anv_bo::size, which may not match the bo's "actual
size" according to the kernel.

Post-patch, the nominal size of the bo returned from
anv_bo_cache_import() is always the size of imported dma-buf according
to lseek(). Pre-patch, the bo's nominal size was difficult to predict.
If the imported dma-buf's gem handle was not resident in the cache, then
the bo's nominal size was align(VkMemoryAllocateInfo::allocationSize,
4096).  If it *was* resident, then the bo's nominal size was whatever
the cache returned. As a consequence, the first cache insert decided the
bo's nominal size, which could be significantly smaller compared to the
dma-buf's actual size, as the nominal size was determined by
VkMemoryAllocationInfo::allocationSize and not lseek().

I believe this patch cleans up that messy behavior. For an imported or
exported VkDeviceMemory, anv_bo::size should now be the true size of the
bo, if I correctly understand the problem (which I possibly don't).

v2:
  - Preserve behavior of aligning size to 4096 before checking. [for
    jekstrand]
  - Check size with < instead of <=, to match behavior of commit c0a4f56f
    "anv: bo_cache: allow importing a BO larger than needed". [for
    chadv]