drm/omap: add CONFIG_MMU dependency

Compile-testing with CONFIG_MMU disabled causes a link error in omapdrm:

arm-linux-gnueabi-ld: drivers/gpu/drm/omapdrm/omap_gem.o: in function `omap_gem_fault_2d':
omap_gem.c:(.text+0x36e): undefined reference to `vmf_insert_mixed'
arm-linux-gnueabi-ld: drivers/gpu/drm/omapdrm/omap_gem.o: in function `omap_gem_fault':
omap_gem.c:(.text+0x74a): undefined reference to `vmf_insert_mixed'

Avoid this by adding a Kconfig dependency.

Fixes: dc6fcaab ("drm/omap: Allow build with COMPILE_TEST=y")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719095942.3841009-1-arnd@kernel.org