drm/shmem-helper: Align to page size in dumb_create

shmem helpers seem a bit sloppy here by automatically rounding up when
actually creating the buffer, which results in under-reporting of what
we actually have. Caught by igt/vgem_basic tests.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603164113.1433476-4-daniel.vetter@ffwll.ch