The driver gets the irq number using platform_get_irq on the main kms
platform device. This works fine since both MDP4 and MDP5 currently
have a flat device hierarchy. The platform device tied with the
drm_device points to the MDP DT node in both cases.

This won't work when MDP5 supports a tree-like hierarchy. In this
case, the platform device tied to the top level drm_device is the
MDSS DT node, and the irq we need for KMS is the one generated by
MDP5, not MDSS.

Get the irq number from the MDP4/5 kms driver itself. Each driver
can later provide the irq number based on what device hierarchy it
uses.

While we're at it, call drm_irq_install only when we have a valid KMS
driver.
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>