drm/vkms: Add information about module options

Update vkms documentation to contain usage of `modinfo`
command and steps to load vkms with module options enabled.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4fbc6459377c1dcbe8e6648718453d5693f6451c.1610391685.git.sylphrenadin@gmail.com