drm/vc4: Use DRM Execution Contexts

VC4 has internal copies of `drm_gem_lock_reservations()` and
`drm_gem_unlock_reservations()` within the driver. Ideally, these
hard-coded functions should be replaced with the generic functions
provided by DRM common code. However, instead of using the DRM GEM
functions to (un)lock reservations, transition to the new DRM Execution
Contexts API.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241220134204.634577-2-mcanal@igalia.com