drm/tidss: Rename 'wait_lock' to 'irq_lock'

The 'wait_lock' name seems to be a copy-paste from omapdrm, and makes no
sense here. Rename it to 'irq_lock'. Also clarify the related comment to
make it clear what it protects, and drop any comments related to
'wait_list' which doesn't exist in tidss.

Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241021-tidss-irq-fix-v1-7-82ddaec94e4a@ideasonboard.com