video: exynos mipi dsi: Properly interpret the interrupt source flags

Rework the interrupt handler so the RX_DONE, FIFO_EMPTY interrupts are
properly detected. This prevents missing the interrupts when there are
other bits set in the INTSRC register than just RX_DONE and FIFO_EMPTY.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Donghwa Lee <dh09.lee@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>