i915_vma: Rename vma_lookup to i915_vma_lookup

Use i915 prefix to avoid name collision with future vma_lookup() in mm.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323134208.3077275-1-Liam.Howlett@Oracle.com