drm/drm_vma_manager.c: Remove useless goto statement

Commit db2395eccf08i ("drm: Convert drm_vma_manager to embedded
interval-tree in drm_mm") removed a line in drm_vma_offset_add() function that
makes checking the result of calling drm_mm_insert_node() and the goto
call redundant. Rework the function (as suggested by Chris Wilson) to
eliminate the need for the goto and associated label.

v2: rewrite function to remove all goto statements.

Fixes: db2395eccf08i ("drm: Convert drm_vma_manager to embedded interval-tree in drm_mm")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101144458.5353-1-Liviu.Dudau@arm.com