drm/i915: ensure segment offset never exceeds allowed max

Commit 255fc170 ("drm/i915/gem: Calculate object page offset for
partial memory mapping") introduced a new offset, which accounts for
userspace mapping not starting from the beginning of object's scatterlist.

This works fine for cases where first object pte is larger than the new
offset - "r->sgt.curr" counter is set to the offset to match the difference
in the number of total pages. However, if object's first pte's size is
equal to or smaller than the offset, then information about the offset
in userspace is covered up by moving "r->sgt" pointer in remap_sg():

	r->sgt.curr += PAGE_SIZE;
	if (r->sgt.curr >= r->sgt.max)
		r->sgt = __sgt_iter(__sg_next(r->sgt.sgp), use_dma(r->iobase));

This means that two or more pages from virtual memory are counted for
only one page in object's memory, because after moving "r->sgt" pointer
"r->sgt.curr" will be 0.

We should account for this mismatch by moving "r->sgt" pointer to the
next pte. For that we may use "r.sgt.max", which already holds the max
allowed size. This change also eliminates possible confusion, when
looking at i915_scatterlist.h and remap_io_sg() code: former has
scatterlist pointer definition, which differentiates "s.max" value
based on "dma" flag (sg_dma_len() is used only when the flag is
enabled), while latter uses sg_dma_len() indiscriminately.

This patch aims to resolve issue:
#12031



v3:
 - instead of checking if r.sgt.curr would exceed allowed max, changed
the value in the while loop to be aligned with `dma` value

v4:
 - remove unnecessary parent relation

v5:
 - update commit message with explanation about page counting mismatch
 and link to the issue

Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/upbjdavlbcxku63ns4vstp5kgbn2anxwewpmnppszgb67fn66t@tfclfgkqijue