drm/vkms: add support for multiple overlay planes

Create 8 overlay planes instead of 1 when the "enable_overlay" module
parameter is set.

The following igt-gpu-tools tests were executed without finding
regressions. Notice than the numbers are identical:

                    |     master branch     |      this  patch      |
                    | SUCCESS | SKIP | FAIL | SUCCESS | SKIP | FAIL |
 kms_atomic         |      10 |   02 |   00 |      10 |   02 |   00 |
 kms_plane_cursor   |      09 |   45 |   00 |      09 |   45 |   00 |
 kms_plane_multiple |      01 |   23 |   00 |      01 |   23 |   00 |
 kms_writeback      |      04 |   00 |   00 |      04 |   00 |   00 |

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220107182809.141003-2-jose.exposito89@gmail.com