drm/vkms: prime import support

Bring dmabuf sharing through implementing prime_import_sg_table callback.
This will help to validate userspace conformance in prime configurations
without using any actual hardware (e.g. in the cloud).

This enables kms_prime IGT testcase on vkms.

V3:
 - Rodrigo: remove redundant vkms_gem_create_private
V2:
 - Rodrigo: styleguide + return code check

Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Simon Ser <simon.ser@intel.com>
Tested-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Signed-off-by: Oleg Vasilev <omrigann@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930155924.21845-1-oleg.vasilev@intel.com