drm/etnaviv: move some functions to a header to be able to use them externally

drm/etnaviv: move some functions to a header to be able to use them externally

v2: Add license info to header

v3: remove unused headers (Christian Gmainer)

[cgmeiner: improve include guard]

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Tested-by: Marek Vasut <marek.vasut@mailbox.org> # STM32MP255C DHCOS DHSBC
Link: https://patch.msgid.link/20251119164624.9297-3-gert.wollny@collabora.com
Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>