drm: aspeed: select CONFIG_DRM_GEM_CMA_HELPER

drm: aspeed: select CONFIG_DRM_GEM_CMA_HELPER

The aspeed driver uses the gem_cma_helper code, but does
noto enforce enabling this through Kconfig:

x86_64-linux-ld: drivers/gpu/drm/aspeed/aspeed_gfx_drv.o:(.rodata+0x2c8): undefined reference to `drm_gem_cma_prime_import_sg_table'
x86_64-linux-ld: drivers/gpu/drm/aspeed/aspeed_gfx_drv.o:(.rodata+0x2d8): undefined reference to `drm_gem_cma_dumb_create'
x86_64-linux-ld: drivers/gpu/drm/aspeed/aspeed_gfx_crtc.o: in function `aspeed_gfx_pipe_update':
aspeed_gfx_crtc.c:(.text+0xe5): undefined reference to `drm_fb_cma_get_gem_obj'

Add the same 'select' that is used in other such drivers.

Fixes: 09717af7d13d ("drm: Remove CONFIG_DRM_KMS_CMA_HELPER option")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211204174637.1160725-2-arnd@kernel.org