drm/i915/dram: add return value and handling to intel_dram_detect()

drm/i915/dram: add return value and handling to intel_dram_detect()

We'll want to start returning errors from intel_dram_detect(). As the
first step, add the return value and error handling, even if we still
only return 0.

Do no functional changes, but leave a comment about whether we should
bail out on dram detection failures.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://lore.kernel.org/r/be2c31c459fb95d8161b719d499403eea5ec17b7.1748337870.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>