drm/vkms: Add setup and testing information

Update the vkms documentation to contain steps to:

 - setup the vkms driver
 - run tests using igt

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209190453.c6kp5winikr55n3i@adolin