drm: aspeed: Clean up Kconfig options

The GFX IP is inside of the ASPEED BMC SoC so there is little use
enabling it on a kernel that does not support ASPEED.

When building with COMPILE_TEST the architecture many not have CMA
support, so to avoid breaking the build we only select these options if
the architecture supports the contiguous allocator.

I suspect the DRM_PANEL came from a cut/paste error.

Fixes: 4f2a8f58

 ("drm: Add ASPEED GFX driver")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405081117.27339-1-joel@jms.id.au