drm/vkms: Suppress context imbalance detected by sparse warning

The functions `vkms_crtc_atomic_begin` and `vkms_crtc_atomic_flush` are
responsible for locking and unlocking a mutex, respectively. Add the
`__acquires` and `__releases` annotations to these functions to prevent
the associated sparse warning about context imbalance.

Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240912-vkms-warnings-v1-1-59f3e13ea8e5@bootlin.com