drm/etnaviv: change return type to vm_fault_t

Use new return type vm_fault_t for fault handler. For
now, this is just documenting that the function returns
a VM_FAULT value rather than an errno. Once all instances
are converted, vm_fault_t will become a distinct type.

Ref- commit 1c8f4220 ("mm: change return type to vm_fault_t")

Previously vm_insert_page() returns err which driver
mapped into VM_FAULT_* type. The new function
vmf_insert_page() will replace this inefficiency by
returning VM_FAULT_* type.

vmf_error() is the newly introduce inline function
in 4.17-rc6.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>