drm/msm/dpu: fix vblank IRQ registration before atomic_mode_set

dpu_encoder_toggle_vblank_for_crtc() can call control_vblank_irq()
at any time in response to a userspace vblank request, independently
of the atomic commit sequence. If this happens before the encoder's
first atomic_mode_set(), irq[INTR_IDX_RDPTR] is still zero.

Passing irq_idx=0 to dpu_core_irq_register_callback() is treated as
invalid, and DPU_IRQ_REG(0) and DPU_IRQ_BIT(0) produce misleading
values of 134217727 and 31 respectively due to unsigned wraparound
in the (irq_idx - 1) macros, resulting in the confusing error:

[dpu error]invalid IRQ=[134217727, 31]

Since irq[INTR_IDX_RDPTR] will be properly populated by
atomic_mode_set() and registered by irq_enable() as part of the
normal modeset sequence, silently skip the vblank IRQ registration
when the index has not yet been initialized. This matches the
existing pattern of the master encoder check above it.

Signed-off-by: Cédric Bellegarde <cedric.bellegarde@adishatz.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/712752/
Link: https://lore.kernel.org/r/20260318171700.394945-1-cedric.bellegarde@adishatz.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>