mtd: MTD_INTEL_DG should depend on DRM_I915 or DRM_XE

Intel Discrete Graphics non-volatile memory is only present on Intel
discrete graphics devices, and its auxiliary device is instantiated by
the Intel i915 and Xe2 DRM drivers.  Hence add dependencies on DRM_I915
and DRM_XE, to prevent asking the user about this driver when
configuring a kernel without Intel graphics support.

Fixes: ceb5ab3c ("mtd: add driver for intel graphics non-volatile memory device")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>