drm/tinydrm: Replace dev_error with DRM_DEV_ERROR

Convert instances of dev_error to DRM_DEV_ERROR as we have
DRM_DEV_ERROR variants of drm print macros.

Signed-off-by: Harsha Sharma <harshasharmaiitr@gmail.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171006221738.30200-1-harshasharmaiitr@gmail.com