intel/ppgtt: memory address alignment

Kernel (for ppgtt) requires memory address to be
aligned to page size (4096).

-v2: added marking that also fixes initial commit 01058a55.
-v3: numbers replaced by PAGE_SIZE; buffer-object size is aligned
instead of alignment of offsets (Chris Wilson).
-v4: changes related to PAGE_SIZE moved to separate commit
-v5: restored alignment to page-size for 0-size.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106997


Fixes: a363bb2c (i965: Allocate VMA in userspace for full-PPGTT systems.)
Fixes: 01058a55 (i965: Add virtual memory allocator infrastructure to brw_bufmgr.)
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit 24839663)