drm/v3d: use new return type vm_fault_t in v3d_gem_fault

Instead of converting an errno into a vm_fault_t ourselves, use
vmf_insert_mixed() which returns a vm_fault_t directly.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180704145556.GA11036@jordon-HP-15-Notebook-PC


Reviewed-by: Matthew Wilcox <willy@infradead.org>