drm/doc: Add VKMS module description and use to "Testing and Validation"

Add a description on VKMS module and the cases in which it should be used.
There's a brief explanation on how to set it and use it in a VM, along with
an example of running an igt-test.

Changes since V3:
 Rodrigo:
 - Change the log message to imperative
 - Fix some bad spelling/writing
 - Add a blank line before enumeration

Changes since V2:
 Andre:
 - Avoid repetition of words in the same sentence;
 - Make the explanation on 'setting the kernel' shorter, eliminate the
   'make menuconfig' command;
 - Add tab on enumeration to have one line per item;
 - Clarify from each machine igt-tests commands should be ran on.

Reviewed-by: Rodrigo Siqueira <[email protected]>
Signed-off-by: Gabriela Bittencourt <[email protected]>
Signed-off-by: Rodrigo Siqueira <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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