drm/mm: Remove redundant assignment in drm_mm_reserve_node

In Pete Goodliffe words, "You can improve a system by adding new code. You
can also improve a system by removing code" - In this case, commit
"202b52b7" added new code to initialize end of the node. So, there
is no need for duplicated initialization, and this patch simply removes it.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200309151156.25040-1-akeem.g.abodunrin@intel.com