drm/exynos/exynos7_drm_decon: Use platform_get_irq_byname() to get the interrupt

drm/exynos/exynos7_drm_decon: Use platform_get_irq_byname() to get the interrupt

platform_get_resource_byname(pdev, IORESOURCE_IRQ, ..) relies on static
allocation of IRQ resources in DT core code, this causes an issue
when using hierarchical interrupt domains using "interrupts" property
in the node as this bypassed the hierarchical setup and messed up the
irq chaining.

In preparation for removal of static setup of IRQ resource from DT core
code use platform_get_irq_byname().

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>