drm: mali-dp: Set the drm->irq_enabled flag to match driver's state.

Mali DP driver does not use drm_irq_{un,}install() function so the
drm->irq_enabled flag does not get set automatically.
drm_wait_vblank() checks the value of the flag among other functions.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>