drm: gem_cma_helper.c: Allow importing of contiguous scatterlists with nents > 1

drm: gem_cma_helper.c: Allow importing of contiguous scatterlists with nents > 1

drm_gem_cma_prime_import_sg_table() will fail if the number of entries
in the sg_table > 1. However, you can have a device that uses an IOMMU
engine and can map a discontiguous buffer with multiple entries that
have consecutive sg_dma_addresses, effectively making it contiguous.
Allow for that scenario by testing the entries in the sg_table for
contiguous coverage.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110133310.1225-1-Liviu.Dudau@arm.com