drm/vkms: change min cursor size to accept smaller values

Change min cursor size of vkms driver from 20 to 10, to increase the IGT
test coverage of vkms by enabling 32x10 cursor size subtests in
kms_cursor_crc

Signed-off-by: Alaa Emad <aemad@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221109113945.20938-1-aemad@igalia.com