drm/panel: Rename GEM CMA helpers GEM DMA helpers

As a result of [1], DRM_GEM_CMA_HELPER has been replaced by
DRM_GEM_CMA_HELPER.

[1] 4a83c26a ("drm/gem: rename GEM CMA helpers to GEM DMA helpers")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>