drm: Fix return type of crc .poll()

Sparse compains:
../drivers/gpu/drm/drm_debugfs_crc.c:350:17: warning: incorrect type in initializer (different base types)
../drivers/gpu/drm/drm_debugfs_crc.c:350:17:    expected restricted __poll_t ( *poll )( ... )
../drivers/gpu/drm/drm_debugfs_crc.c:350:17:    got unsigned int ( * )( ... )

Change the .poll() return type to __poll_t to silence it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125143.9965-5-ville.syrjala@linux.intel.com
Reviewed-by: Sean Paul <sean@poorly.run>