drm/komeda: Make Komeda interrupts shareable

Komeda interrupts may be shared with other hardware blocks.
One needs to use devm_request_irq() with IRQF_SHARED to create a shared
 interrupt handler.
As a result of not using drm_irq_install() api, one needs to set
"(struct drm_device *)->irq_enabled = true/false" to enable/disable
vblank interrupts.

Changes from v1:-
1. Squashed the following two patches into one (as the second patch is a
consequence of the first one):-
   drm/komeda: Avoid using DRIVER_IRQ_SHARED
   drm/komeda: Enable/Disable vblank interrupts
2. Fixed the commit message (as pointed by Daniel Vetter)
3. Removed calls to 'drm_irq_uninstall()' as we are no longer using
drm_irq_install()
4. Removed the struct member 'komeda_kms_driver.irq_handler' as it is not
used anywhere.

Signed-off-by: Ayan Halder <ayan.halder@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>