Full rename without any modification, except to the Makefile.

Renaming ingenic-drm.c to ingenic-drm-drv.c allow to decouple the module
name from the source file name in the Makefile. This will be useful
later when more source files are added.

v2: New patch
v3: No change
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-4-paul@crapouillou.net