video: fbdev: Validate mode timing against monspec

fbmon may generate mode timings that are out of spec of the monitor.
eg DELL U2410 has a max clock 170mhz but advertises a resolutions of
1920x1200@60 in its Standard Timings using 2byte code of D1 00.
When this is looked up in the DMT table it gives it a 193mhz clock.
Although the DELL monitor supports 1920x1200@60, it can only run with
reduced timings at 154mhz or DMT id 0x44 which has no STD 2byte code.
This patch checks to see if the mode can be supported by the monitor
by comparing against monspecs.dclkmax.

Signed-off-by: David Ung <davidu@nvidia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>