drm: Attempt to parse SPI devices as platform bus devices.

For ARM systems with tinydrm displays attached to SPI, the bus name is
/spi but we have platform device info for the rest.  Fixes
eglInitialize() failures on hx8357d since the EGL_EXT_device_drm
changes.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>